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FRAGMENTS  OP  SCIENCE 

VOL.  I 


FEAaMENTS    OF 

SCIEE'CE 


A  SERIES  OF  DETACHED 

ESSAYS,  ADDRESSES,  AND  REVIEWS 


BY 

JOHN  TYNDALL,  F.R.S. 

AUTHOR   OF   NEW    FRAGMENTS,    HEAT   AS   A   MODE    OF   MOTION, 

ON    SOUND,   RADIANT   HEAT,   ON   FORMS   OF   WATER, 

HOURS   OF   EXERCISE   IN   THE   ALPS,   ETC, 


VOL.  I 


NEW    YORK 

D.   APPLETON   AND   COMPANY 

1897 


Authorized  Edition^ 


PUBLISHEKS'  KOTE. 


The  first  edition  of  Prof.  TyndalFs  "  Fragments  of 
Science  "  was  published  some  twenty  years  ago  as  a  sin- 
gle volume,  which  was  made  up  of  a  score  or  more  of 
his  detached  essays,  addresses,  and  reviews.  The  book 
was  afterward  revised,  some  of  the  papers  recast,  and 
from  time  to  time  new  ones  added,  until,  the  size  of  the 
work  becoming  somewhat  unwieldy,  the  present  two- 
volume  edition  was  decided  upon.  This  contains  fifteen 
additional  papers,  and  represents  the  author's  latest 
changes  and  revisions.  The  volumes  are  uniform  with 
"New  Fragments,"  recently  issued,  and  the  three  to- 
gether include  all  the  occasional  writings  which  their 
author  has  decided  to  preserve  in  permanent  form. 


PEEFAOE 

TO 

THE    SIXTH    EDITION, 


To  AVOID  unwieldiness  of  bulk  this  edition  of  the 
'  Fragments '  is  published  in  two  volumes,  instead  of, 
as  heretofore,  in  one. 

The  first  volume  deals  almost  exclusively  with  the 
laws  and  phenomena  of  matter.  The  second  trenches 
upon  questions  in  which  the  phenomena  of  matter  in- 
terlace more  or  less  with  those  of  mind. 

New  Essays  have  been  added,  while  old  ones  have 
been  revised,  and  in  part  recast.  To  be  clear,  without 
being  superficial,  has  been  my  aim  throughout. 

In  neither  volimie  have  I  aspired  to  sit  in  the  seat 
of  the  scornful,  but  rather  to  treat  the  questions  touched 
upon  with  a  tolerance,  if  not  a  reverence,  befitting  their 
difficulty  and  weight. 

Holding,  as  I  do,  the  nebular  hypothesis,  I  am  logi- 
cally bound  to  deduce  the  life  of  the  world  from  forces 


viii  PREFACE. 

inherent  in  the  nebula.  With  this  view,  which  is  set 
forth  in  the  second  volume,  it  seemed  but  fair  to  asso- 
ciate the  reasons  which  cause  me  to  conclude  that  every 
attempt  made  in  our  day  to  generate  life  independently 
of  antecedent  life  has  utterly  broken  down. 

A  discourse  on  the  Electric  Light  winds  up  the 
second  volume.  The  incongruity  of  its  position  is  to 
be  referred  to  the  lateness  of  its  delivery. 


CONTENTS 


OF 


THE    FIKST    VOLUME 


OHAFTKR  TAGia 

I.  THE  CONSTITUTION   OP   NATURE               »               •  •          3 

n.  RADIATION      ,               •               .                .               .               ,  .        28 

in.  ON    RADIANT   HEAT    IN    RELATION    TO    THE    COLOUR 

AND   CHEMICAL   CONSTITUTION   OF   BODIES  .        74 

IV.  NEW  CHEMICAL  REACTIONS   PRODUCED  BY   LIGHT  .        96 

V.  THE  SKY   ,               ,               .               .               .               .  .131 
VI.  VOYAGE   TO  ALGERIA   TO   OBSEUVE   THE   ECLIPSE  ,      142 

VII.  NIAGARA   .               .               .               .               .               •  ,175 

VIII.  THE   PARALLEL  ROADS  OP  GLEN   ROY          ,               •  .     205 

IX.  ALPINE  SCULPTURE           .               .               .               •  ,229 

X.  RECENT  EXPERIMENTS  ON  POG-SIGNALS     •               ,  ,      253 

XI.  ON  THE  STUDY  OP  PHYSICS         .               •               •  •     281 

XII.  ON  CRYSTALLINE  AND  SLATY  CLEAVAGE   .               .  .     304 

XIII.  ON    PARAMAGNETIC  AND   DIAMAGNETIC   lOUCES  .     321 

XIV.  PHYSICAL  BASIS   OF  SOLAR  CHEMISTRY       .               ,  ,     829 
XV.  ELEMENTARY  MAGNETISM              •              •               •  .     343 

XVI.  ON   FORCE         .               .               ,               ,               ,               ,  ,     369 

XVII.  COl^TRIBUTIONS  TO  MOLEOULAB  PHYSICS           •  .     386 


X  CONTENTS. 

CHAPTEB  PAQK 

XVIII.  LIFE  AND  LBTTBBS  OP  FARADAY.  ,               ,  .     399 

XIX.  THE  COPLEY  MEDALLIST   OP   1870  ,               ,  ,     422 

XX.  THE   COPLEY  MEDALLIST  OP  1871  .  •              ,  ,     429 

XXI.  DEATH  BY  LIGHTNING  .               I  ,               ,  .439 

XXII.  SCIENCE  AND  THE  SPIBITS                •  •               •  .     444 


MAP 

SHOWING  THE  PABALLEL  ROADS  OF  GLEN  BOY       •     tofocep.  227 


VOL.  I. 

INORGANIC    NATURE 


I. 

TEE   CONSTITUTION  OF  NATURE.'-^ 

WE  cannot  think  of  space  as  finite,  for  wherever 
in  imagination  we  erect  a  boundary,  we  are  com- 
pelled to  think  of  space  as  existing  beyond  it.  Thus 
by  the  incessant  dissolution  of  limits  we  arrive  at  a 
more  or  less  adequate  idea  of  the  infinity  of  space. 
But,  though  compelled  to  think  of  space  as  unbounded, 
there  is  no  mental  necessity  compelling  us  to  think  of 
it  either  as  filled  or  empty ;  whether  it  is  so  or  not  must 
be  decided  by  experiment  and  observation.  That  it  is 
not  entirely  void,  the  starry  heavens  declare  ;  but  the 
question  still  remains,  Are  the  stars  themselves  hung 
in  vacuo  ?  Are  the  vast  regions  which  surround  them, 
and  across  which  their  light  is  propagated,  absolutely 
empty  ?  A  century  ago  the  answer  to  this  question, 
founded  on  the  Newtonian  theory,  would  have  been, 
*  No,  for  particles  of  light  are  incessantly  shot  through 
space.'  The  reply  of  modern  science  is  also  negative, 
but  on  difierent  grounds.  It  has  the  best  possible 
reasons  for  rejecting  the  idea  of  luminiferous  particles  ; 
but,  in  support  of  th**  conclusion  that  the  celestial 
spaces  are  occupied  by  matter,  it  is  able  to  ofier  proofs 

»  *Fortnightl7  Beview,*  1865,  voL  iii.  p.  129. 


4  FRAGMENTS  OF  SCIENCE. 

almost  as  cogent  as  those  which  can  be  adduced  of  the 
existence  of  an  atmosphere  round  the  earth.  Men's 
minds,  indeed,  rose  to  a  conception  of  the  celestial  and 
universal  atmosphere  through  the  study  of  the  terres- 
trial and  local  one.  From  the  phenomena  of  sound, 
as  displayed  in  the  air,  they  ascended  to  the  phenomena 
of  light,  as  displayed  in  the  ether ;  which  is  the  name 
given  to  the  interstellar  medium. 

The  notion  of  this  medium  must  not  be  considered 
as  a  vague  or  fanciful  conception  on  the  part  of  scientific 
men.  Of  its  reality  most  of  them  are  as  convinced 
as  they  are  of  the  existence  of  the  sun  and  moon.  The 
luminiferous  ether  has  definite  mechanical  properties. 
It  is  almost  infinitely  more  attenuated  than  any  known 
gas,  but  its  properties  are  those  of  a  solid  rather  than 
of  a  gas.  It  resembles  jelly  rather  than  air.  This 
was  not  the  first  conception  of  the  ether,  but  it  is  that 
forced  upon  us  by  a  more  complete  knowledge  of  its 
phenomena.  A  body  thus  constituted  may  have  its 
boundaries ;  but,  although  the  ether  may  not  be  co- 
extensive with  space,  it  must  at  all  events  extend  as  far 
as  the  most  distant  visible  stars.  In  fact  it  is  the 
vehicle  of  their  light,  and  without  it  they  could  not 
be  seen.  This  all-pervading  substance  takes  up  their 
molecular  tremors,  and  conveys  them  with  inconceivable 
rapidity  to  our  organs  of  vision.  It  is  the  transported 
shiver  of  bodies  countless  millions  of  miles  distant, 
which  translates  itself  in  human  consciousness  into  the 
splendour  of  the  firmament  at  night. 

If  the  ether  have  a  boundary,  masses  of  ponderable 
matter  might  be  conceived  to  exist  beyond  it,  but  they 
could  emit  no  light.  Beyond  the  ether  dark  suns 
might  bum ;  there,  under  proper  conditions,  combustion 
might  be  carried  on;  fuel  might  consume  unseen,  and 
metals  be  fused  in  invisible  fires.     A  body,  moreover, 


THE  CONSTITUTION  OF  NATUEE.  6 

once  heated  there,  would  continue  for  ever  heated ;  a 
sun  or  planet  once  molten,  would  continue  for  ever 
molten.  For,  the  loss  of  heat  being  simply  the  ab- 
straction of  molecular  motion  by  the  ether,  where  this 
medium  is  absent  no  cooling  could  occur.  A  sentient 
being  on  approaching  a  heated  body  in  this  region, 
would  be  conscious  of  no  augmentation  of  temperature. 
The  gradations  of  warmth  dependent  on  the  laws  of 
radiation  would  not  exist,  and  actual  contact  would 
first  reveal  the  heat  of  an  extra  ethereal  sun. 

Imagine  a  paddle-wheel  placed  in  water  and  caused 
to  rotate.  From  it,  as  a  centre,  waves  would  issue  in 
all  directions,  and  a  wader  as  he  approached  the  place 
of  disturbance  would  be  met  by  stronger  and  stronger 
waves.  This  gradual  augmentation  of  the  impression 
made  upon  the  wader  is  exactly  analogous  to  the  aug- 
mentation of  light  when  we  approach  a  luminous  source. 
In  the  one  case,  however,  the  coarse  common  nerves  of 
the  body  suffice ;  for  the  other  we  must  have  the  finer 
optic  nerve.  But  suppose  the  water  withdrawn;  the 
action  at  a  distance  would  then  cease,  and,  as  far  as  the 
sense  of  touch  is  concerned,  the  wader  would  be  first 
rendered  conscious  of  the  motion  of  the  wheel  by  the 
blow  of  the  paddles.  The  transference  of  motion  from 
the  paddles  to  the  water  is  mechanically  similar  to 
the  transference  of  molecular  motion  from  the  heated 
body  to  the  ether;  and  the  propagation  of  waves 
through  the  liquid  is  mechanically  similar  to  the  pro- 
pagation of  light  and  radiant  heat. 

As  far  as  our  knowledge  of  space  extends,  we  are 
to  conceive  it  as  the  holder  of  the  luminiferous  ether, 
through  which  are  interspersed,  at  enormous  distances 
apart,  the  ponderous  nuclei  of  the  stars.  Associated 
with  the  star  that  most  concerns  us  we  have  a  group 
of  dark  planetary  masses  revolving  at  various  distance^ 


6  FRAGMENTS  OF  SCIENCE. 

round  it,  each  again  rotating  on  its  ot^ti  axis ;  and, 
finally,  associated  with  some  of  these  planets  we  have 
dark  bodies  of  minor  note — the  moons.  Whether  the 
other  fixed  stars  have  similar  planetary  companions  or 
not  is  to  us  a  matter  of  pure  conjecture,  which  may  or 
may  not  enter  into  our  conception  of  the  universe.  But 
probably  every  thoughtful  person  believes,  with  regard 
to  those  distant  suns,  that  there  is,  in  space,  something 
besides  our  system  on  which  they  shine. 

From  this  general  view  of  the  present  condition  of 
space,  and  of  the  bodies  contained  in  it,  we  pass  to 
the  enquiry  whether  things  were  so  created  at  the  be- 
ginning. Was  space  furnished  at  once,  by  the  fiat  of 
Omnipotence,  with  these  burning  orbs  ?  In  presence 
of  the  revelations  of  science  this  view  is  fading  more 
and  more.  Behind  the  orbs,  we  now  discern  the  nebulae 
from  which  tbey  have  been  condensed.  And  without 
going  so  far  back  as  the  nebulae,  the  man  of  science  can 
prove  that  out  of  common  non-luminous  matter  this 
whole  pomp  of  stars  might  have  been  evolved. 

The  law  of  gravitation  enunciated  by  Newton  is, 
that  every  particle  of  matter  in  the  universe  attracts 
every  other  particle  with  a  force  which  diminishes  as 
the  square  of  the  distance  increases.  Thus  the  sun  and 
the  earth  mutually  pull  each  other ;  thus  the  earth  and 
the  moon  are  kept  in  company  ;  the  force  which  holds 
every  respective  pair  of  masses  together  being  the  in- 
tegrated force  of  their  component  parts.  Under  the 
operation  of  this  force  a  stone  falls  to  the  ground  and 
is  warmed  by  the  shock ;  under  its  operation  meteors 
plunge  into  our  atmosphere  and  rise  to  incandescence. 
Showers  of  such  meteors  doubtless  fall  incessantly  upon 
the  sun.  Acted  on  by  this  force,  the  earth,  were  it 
stopped,  in  its  orbit  to-morrow,  would  rush  towards,  and 
finally  combine  with,  the  sun.      Heat  would  also  be 


THE  CONSTITUTION  OF  NATUBE.  7 

developed  by  this  collision.  Mayer  first,  and  Helm- 
holtz  and  Thomson  afterwards,  have  calculated  ita 
amount.  It  would  equal  that  produced  by  the  com- 
bustion of  more  than  5,000  worlds  of  solid  coal,  all 
this  heat  being  generated  at  the  instant  of  collision. 
In  the  attraction  of  gravity,  therefore,  acting  upon 
non-luminous  matter,  we  have  a  source  of  heat  more 
powerful  than  could  be  derived  from  any  terrestrial 
combustion.  And  were  the  matter  of  the  universe 
thrown  in  cold  detached  fragments  into  space,  and  there 
abandoned  to  the  mutual  gravitation  of  its  own  parts, 
the  collision  of  the  fragments  would  in  the  end  pro- 
duce the  fires  of  the  stars. 

The  action  of  gravity  upon  matter  originally  cold 
may,  in  fact,  be  the  origin  of  all  light  and  heat,  and 
also  the  proximate  source  of  such  other  powers  as  are 
generated  by  light  and  heat.  But  we  have  now  to 
enquire  what  is  the  light  and  what  is  the  heat  thus 
produced  ?  This  question  has  already  been  answered 
in  a  general  way.  Both  light  and  heat  are  modes  of 
motion.  Two  planets  clash  and  come  to  rest ;  their 
motion,  considered  as  that  of  masses,  is  destroyed,  but 
it  is  in  great  part  continued  as  a  motion  of  their 
ultimate  particles.  It  is  this  latter  motion,  taken  up 
by  the  ether,  and  propagated  through  it  with  a  velo- 
city of  186,000  miles  a  second,  that  comes  to  us  as 
the  light  and  heat  of  suns  and  stars.  The  atoms  of  a 
hot  body  swing  with  inconceivable  rapidity — billions  of 
times  in  a  second — but  this  power  of  vibration  neces- 
sarily implies  the  operation  of  forces  between  the 
atoms  themselves.  It  reveals  to  us  that  while  they 
are  held  together  by  one  force,  they  are  kept  asunder 
by  another,  tlieir  position  at  any  moment  depending 
on  the  equilibrium  of  attraction  and  repulsion.  The 
atoms  behave  as  if  connected  by  elastic  springs,  which 
2 


8  FRAGMENTS  OF  SCIENCE. 

oppose  at  the  same  time  their  approach  and  their 
retreat,  but  which  tolerate  the  vibration  called  heat. 
The  molecular  vibration  once  set  up  is  instantly  shared 
^vith  the  ether,  and  diffused  by  it  throughout  space. 

We  on  the  earth's  surface  live  night  and  day  in  the 
midst  of  ethereal  commotion.  The  medium  is  never 
still.  The  cloud  canopy  above  us  may  be  thick  enough. 
to  shut  out  the  light  of  the  stars ;  but  this  canopy  is 
itself  a  warm  body,  which  radiates  its  thermal  motion 
through  the  ether.  The  earth  also  is  warm,  and  sends 
its  heat-pulses  incessantly  forth.  It  is  the  waste  of  its 
molecular  motion  in  space  that  chills  the  earth  upon  a 
clear  night ;  it  is  the  return  of  thermal  motion  from 
the  clouds  which  prevents  th«  earth's  temperature, 
on  a  cloudy  night,  from  falliog  so  low.  To  the  con- 
ception of  space  beiug  filled,  we  must  therefore  add 
the  conception  of  its  being  in  a  state  of  incessant 
tremor. 

The  sources  of  this  vibration  are  the  ponderable 
masses  of  the  universe.  Let  us  take  a  sample  of  these 
and  examine  it  in  detail.  When  we  look  to  our  planet, 
we  find  it  to  be  an  aggregate  of  solids,  liquids,  and 
gases.  Subjected  to  a  sufficiently  low  temperature,  the 
two  last  would  also  assume  the  solid  form.  When  we 
look  at  any  one  of  these,  we  generally  find  it  composed 
of  still  more  elementary  parts.  We  learn,  for  example, 
that  the  water  of  our  rivers  is  formed  by  the  union,  in 
definite  proportions,  of  two  gases,  oxygen  and  hydrogen. 
We  know  how  to  bring  these  constituents  together,  so 
as  to  form  water  :  we  also  know  how  to  analyse  the  water, 
and  recover  from  it  its  two  constituents.  So,  likewise, 
as  regards  the  solid  portions  of  the  earth.  Our  chalk 
hills,  for  example,  are  formed  by  a  combination  of  car- 
bon, oxygen,  and  calcium.  These  are  the  so-called 
dements  the  union  of  which,  in  definite  proportions,  has 


THE   CONSTITUTION   OF  NATUKE.  9 

resulted  in  the  formation  of  chalk.  The  flints  within 
the  chalk  we  know  to  be  a  compound  of  oxygen  and 
silicium,  called  silica ;  and  our  ordinary  clay  is,  for  the 
most  part,  formed  by  the  union  of  silicium,  oxygen,  and 
the  well-known  light  metal,  aluminium.  By  far  the 
greater  portion  of  the  earth's  crust  is  compounded 
of  the  elementary  subs^^ances  mentioned  in  these  few 
lines. 

The  principle  of  gravitation  has  been  already  de- 
scribed as  an  attraction  which  every  particle  of  matter, 
however  small,  exerts  on  every  other  particle.  With 
gravity  there  is  no  selection  ;  no  particular  atoms  choose, 
by  preference,  other  particular  atoms  as  objects  of  attrac- 
tion ;  the  attraction  of  gravitation  is  proportional  simply 
to  the  quantity  of  the  attracting  matter,  regardless  of 
its  quality.  But  in  tlie  molecular  world  which  we  have 
now  entered  matters  are  otherwise  arranged.  Here  we 
have  atoms  between  which  a  strong  attraction  is  exer- 
cised, and  also  atoms  between  which  a  weak  attraction 
IS  exercised.  One  atom  can  jostle  another  out  of  its  place, 
in  virtue  of  a  superior  force  of  attraction.  But,  though 
the  amount  of  force  exerted  varies  thus  from  atom  to 
atom,  it  is  still  an  attraction  of  the  same  mechanical 
quality,  if  I  may  use  the  term,  as  that  of  gravity  itself. 
Its  intensity  might  be  measured  in  the  same  way,  namely 
by  the  amount  of  motion  which  it  can  generate  in  a 
certain  time.  Thus"  the  attraction  of  gravity  at  the 
earth's  surface  is  expressed  by  the  number  32  ;  because, 
when  acting  freely  on  a  body  for  a  second  of  time,  gra- 
vity imparts  to  the  body  a  velocity  of  thirty-two  feet  a 
second.  In  like  manner  the  mutual  attraction  of  oxygen 
and  hydrogen  might  be  measured  by  the  velocity  im- 
parted to  the  atoms  in  their  rushing  together.  Of  course 
Buch  a  unit  of  time  as  a  second  is  not  here  to  be  thought 
of,  the  whole  interval  required  by  the  atoms  to  cross  the 


10  FRAGMENTS  OF  SCIENCE. 

minute  spaces  which  separate  them  amounting  only  to 
an  inconceivably  small  fraction  of  a  second. 

It  has  been  stated  that  when  a  body  falls  to  the 
earth  it  is  warmed  by  the  shock.  Here,  to  use  the  ter- 
minology of  Mayer,  we  have  a  mechanical  combination 
of  the  earth  and  the  body.  Let  us  suffer  the  falling 
body  and  the  earth  to  dwindle  in  imagination  to  the 
size  of  atoms,  and  for  the  attraction  of  gravity  let  us 
substitute  that  of  chemical  affinity ;  we  have  then  what 
is  called  a  chemical  combination.  The  effect  of  the 
union  in  this  case  also  is  the  development  of  heat,  and 
from  the  amount  of  heat  generated  we  can  infer  the 
intensity  of  the  atomic  pull.  Measured  by  ordinary 
mechanical  standards,  this  is  enormous.  Mix  eight 
pounds  of  oxygen  with  one  of  hydrogen,  and  pass  a  spark 
through  the  mixture  ;  the  gases  instantly  combine,  their 
atoms  rushing  over  the  little  distances  which  separate 
them.  Take  a  weight  of  47,000  pounds  to  an  elevation  of 
1,000  feet  above  the  earth's  surface,  and  let  it  fall;  the 
energy  with  which  it  will  strike  the  earth  will  not  exceed 
that  of  the  eight  pounds  of  oxygen  atoms,  as  they 
dash  against  one  pound  of  hydrogen  atoms  to  form  water. 

It  is  sometimes  stated  that  gravity  is  distinguished 
from  all  other  forces  by  the  fact  of  its  resisting  conver- 
sion into  other  forms  of  force.  Chemical  affinity,  it  is 
said,  can  be  converted  into  heat  and  light,  and  these 
again  into  magnetism  and  electricity  :  but  gravity  re- 
fuses to  be  so  converted ;  being  a  force  maintaining 
itself  under  all  circumstances,  and  not  capable  of  dis- 
appearing to  give  place  to  another.  The  statement 
arises  from  vagueness  of  thought.  If  by  it  be  meant 
that  a  particle  of  matter  can  never  be  deprived  of  its 
weight,  the  assertion  is  correct ;  but  the  law  which 
affirms  the  convertibility  of  natural  forces  was  never 
intended,  in  the  minds  of  those  who  understood  it,  to 


THE  CONSTITUTION  OP  NATUEB.  ^   11 

affirm  that  such  a  conversion  as  that  here  implied  occurs 
in  any  case  whatever.  As  regards  convertibility  into 
heat,  gravity  and  chemical  affinity  stand  on  precisely  the 
same  footing.  The  attraction  in  the  one  case  is  as  in- 
destructible as  in  the  other.  Nobody  affirms  that  when 
a  stone  rests  upon  the  surface  of  the  earth,  the  mutual 
attraction  of  the  earth  and  stone  is  abolislied ;  nobody 
means  to  affirm  that  the  mutual  attraction  of  oxygen 
for  hydrogen  ceases,  after  the  atoms  have  combined  to 
form  water.  What  is  meant,  in  the  case  of  chemical 
affinity,  is,  that  the  pull  of  that  affinity,  acting  through 
a  certain  space,  imparts  a  motion  of  translation  of  the 
one  atom  towards  the  other.  This  motion  is  not  heat, 
nor  is  the  force  that  produces  it  heat.  But  when  the 
atoms  strike  and  recoU,  the  motion  of  translation  is  con- 
verted into  a  motion  of  vibration,  which  is  heat.  The 
vibration,  however,  so  far  from  causing  the  extinction  of 
the  original  attraction,  is  in  part  carried  on  by  that 
attraction.  The  atoms  recoil,  in  virtue  of  the  elastic 
force  which  opposes  actual  contact,  and  in  the  recoil 
they  are  driven  too  far  back.  The  originahattraction 
then  triumphs  over  the  force  of  recoil,  and  urges  the 
atoms  once  more  together.  Thus,  like  a  pendulum, 
they  oscillate,  until  their  motion  is  imparted  to  the 
surrounding  aether ;  or,  in  other  words,  until  their  heat 
becomes  radiant  heat. 

In  this  sense,  and  in  this  sense  only,  is  chemical 
affinity  converted  into  heat.  There  is,  first  of  all,  the 
attraction  between  the  atoms  ;  there  is,  secondly,  space 
between  them.  Across  this  space  the  attraction  urges 
them.  They  collide,  they  recoil,  they  oscillate.  There 
is  here  a  change  in  the  form  of  the  motion,  but  there  is 
no  real  loss.  It  is  so  with  the  attraction  of  gravity. 
To  produce  motion  by  gravity  space  must  also  intervene 
between  the  attracting  bodies.     When  they  strike  to- 


12  FRAGMENTS  OF  SCIENCE. 

gether  motion  is  apparently  destroyed,  but  in  realitj/ 
there  is  no  destruction.  Their  atoms  are  suddenly  urged 
together  by  the  shock ;  by  their  own  perfect  elasticity 
these  atoms  recoil ;  and  thus  is  set  up  the  molecular 
oscillation  which,  when  communicated  to  the  proper 
nerves,  announces  itself  as  heat. 

It  was  formerly  universally  supposed  that  by  the  col- 
lision of  unelastic  bodies  force  was  destroyed.  Men  saw, 
for  example,  that  when  two  spheres  of  clay,  painter's 
putty,  or  lead  for  example,  were  urged  together,  the 
motion  possessed  by  the  masses,  prior  to  impact,  was 
more  or  less  annihilated.  They  believed  in  an  absolute 
destruction  of  the  force  of  impact.  Until  recent  times, 
indeed,  no  difficulty  was  experienced  in  believing  this, 
whereas,  at  present,  the  ideas  of  force  and  its  destruc- 
tion refuse  to  be  united  in  most  philosophic  minds.  In 
the  collision  of  elastic  bodies,  on  the  contrary,  it  was 
observed  that  the  motion  with  which  they  clashed  to- 
gether was  in  great  part  restored  by  the  resiliency  of 
the  masses,  the  more  perfect  the  elasticity  the  more 
complete  being  the  restitution.  This  led  to  the  idea  of 
perfectly  elastic  bodies — bodies  competent  to  restore  by 
their  recoil  the  whole  of  the  motion  which  they  possessed 
before  impact — and  this  again  to  the  idea  of  the  con- 
servation of  force,  as  opposed  to  that  destruction  of 
force  which  was  supposed  to  occur  when  unelastic  bodies 
met  in  collision. 

We  now  know  that  the  principle  of  conservation 
holds  equally  good  with  elastic  and  unelastic  bodies.  Per- 
fectly elastic  bodies  would  develop  no  heat  on  collision. 
They  would  retain  their  motion  afterwards,  though  its 
direction  might  be  changed ;  and  it  is  only  when  sensible 
motion  is  wholly  or  partly  destroyed,  that  heat  is  gene- 
rated. This  always  occurs  in  unelastic  collision,  the 
heat  developed  being  the  exact  equivalent  of  the  sensible 


THE  CONSTITUTION  OF  NATUEE.  -13 

motion  extinguished.  This  heat  virtually  declares  that 
the  property  of  elasticity,  denied  to  the  masses,  exists 
among  their  atoms  ;  by  the  recoil  and  oscillation  of 
which  the  principle  of  conservation  is  vindicated. 

But  ambiguity  in  the  use  of  the  term  '  force '  makes 
itself  more  and  more  felt  as  we  proceed.  We  have 
called  the  attraction  of  gravity  a  force,  without  any 
reference  to  motion.  A  body  resting  on  a  shelf  is  as 
much  puJled  by  gravity  as  when,  after  having  been 
pushed  off  the  shelf,  it  falls  towards  the  earth.  We 
applied  the  term  force  also  to  that  molecular  attraction 
which  we  called  chemical  affinity.  When,  however,  we 
spoke  of  the  conservation  of  force,  in  the  case  of  elastic 
collision,  we  meant  neither  a  pull  nor  a  push,  which,  as 
just  indicated,  might  be  exerted  upon  inert  matter,  but 
we  meant  force  invested  in  motion — the  via  viva,  as  it 
IS  called,  of  the  colliding  masses. 

Force  in  this  form  has  a  definite  mechanical  mea- 
sure, in  the  amount  of  work  that  it  can  perform.  The 
simplest  form  of  work  is  the  raising  of  a  weight.  A 
man  walking  up-hill,  or  up-stairs,  with  a  pound  weight 
in  his  hand,  to  an  elevation  say  of  sixteen  feet,  performs 
a  certain  amount  of  work,  over  and  above  the  lifting  of 
his  own  body.  If  he  carries  the  pound  to  a  height  of 
thirty-two  feet,  be  does  twice  the  work;  if  to  a  height 
of  forty-eight  feet,  he  does  three  times  the  work ;  if  to 
sixty-four  feet,  he  does  four  times  the  work,  and  so  on. 
If,  moreover,  he  carries  up  two  poimds  instead  of  one, 
other  things  being  equal,  he  does  twice  the  work ;  if 
three,  four,  or  five  pounds,  he  does  three,  four,  or  five 
times  the  work.  In  fact,  it  is  plain  that  the  work  per- 
formed depends  on  two  factors,  the  weight  raised  and 
the  height  to  which  it  is  raised.  It  is  expressed  by  the 
product  of  these  two  factors. 

But  a  body  may  be  caused  to  reach  a  certain  elevar 


14  FEAGMENTS  OF  SCIENCE. 

tion  in  opposition  to  the  force  of  gravity,  without  being 
actually  carried  up.  If  a  hodman,  for  example,  wished 
to  land  a  brick  at  an  elevation  of  sixteen  feet  above  the 
place  where  he  stood,  he  would  probably  pitch  it  up  to 
the  bricklayer.  He  would  thus  impart,  by  a  sudden 
effort,  a  velocity  to  the  brick  sufficient  to  raise  it  to  the 
required  height ;  the  work  accomplished  by  that  effort 
being  precisely  the  same  as  if  he  had  slowly  carried 
up  the  brick.  The  initial  velocity  to  be  imparted,  in 
this  case,  is  well  known.  To  reach  a  height  of  sixteen 
feet,  the  brick  must  quit  the  man's  hand  with  a  velocity 
of  thirty-two  feet  a  second.  It  is  needless  to  say,  that 
a  body  starting  with  any  velocity,  would,  if  wholly  un- 
opposed or  unaided,  continue  to  move  for  ever  with  the 
same  velocity.  But  when,  as  in  the  case  before  us,  the 
body  is  thrown  upwards,  it  moves  in  opposition  to  gra- 
vity, which  incessantly  retards  its  motion,  and  finally 
brings  it  to  rest  at  an  elevation  of  sixteen  feet.  If  not 
here  caught  by  the  bricklayer,  it  would  return  to  the 
hodman  with  an  accelerated  motion,  and  reach  his  hand 
with  the  precise  velocity  it  possessed  on  quitting  it. 

An  important  relation  between  velocity  and  work  is 
here  to  be  pointed  out.  Supposing  the  hodman  com- 
petent to  impart  to  the  brick,  at  starting,  a  velocity  of 
sixty-four  feet  a  second,  or  twice  its  former  velocity, 
would  the  amount  of  work  performed  be  twice  what  it 
was  in  the  first  instance  ?  No  ;  it  would  be  four  times 
that  quantity  ;  for  a  body  starting  with  twice  the  velocity 
of  another,  will  rise  to  four  times  the  height.  In  like 
manner,  a  three-fold  velocity  will  give  a  nine-fold  eleva- 
tion, a  four-fold  velocity  will  give  a  sixteen-fold  elevation, 
and  so  on.  The  height  attained,  then,  is  not  propor- 
tional to  the  initial  velocity,  but  to  the  square  of  the 
velocity.  As  before,  the  work  is  also  proportional  to 
the  weight  elevated.    Hence  the  work  which  any  moving 


THE  CONSTITUTION   OF  NATUKE.  16 

mass  wliatever  is  competent  to  perform,  in  virtue  of  the 
motion  which  it  at  any  moment  possesses,  is  jointly  pro- 
portional to  its  weight  and  the  square  of  its  velocity. 
Here,  then,  we  have  a  second  measure  of  work,  in  which 
we  simply  translate  the  idea  of  height  into  its  equivalent 
idea  of  motion. 

In  mechanics,  the  product  of  the  mass  of  a  moving 
body  into  the  square  of  its  velocity,  expresses  what  is 
called  the  vis  viva,  or  living  force.  It  is  also  sometimes 
called  the  '  mechanical  effect.'  If,  for  example,  a  cannon 
pointed  to  the  zenith  urge  a  ball  upwards  with  twice 
the  velocity  imparted  to  a  second  ball,  the  former  will 
rise  to  four  times  the  height  attained  by  the  latter.  If 
directed  against  a  target,  it  will  also  do  four  times  the 
execution.  Hence  the  importance  of  imparting  a  high 
velocity  to  projectiles  in  war.  Having  thus  cleared  our 
way  to  a  perfectly  definite  conception  of  the  via  viva  of 
moving  masses,  we  are  prepared  for  the  announcement 
that  the  heat  generated  by  the  shock  of  a  falling  body 
against  the  earth  is  proportional  to  the  vis  mva  annihil- 
ated. The  heat  is  proportional  to  the  square  of  the 
velocity.  In  the  case,  therefore,  of  two  cannon-balls  of 
equal  weight,  if  one  strike  a  target  with  twice  the  velo- 
city of  the  other,  it  will  generate  four  times  the  heat, 
if  with  thi-ee  times  the  velocity,  it  will  generate  nine 
times  the  heat,  and  so  on. 

Mr.  Joule  has  shown  that  a  pound  weight  falling  from 
a  height  of  772  feet,  or  772  pounds  falling  through  one 
foot,  will  generate  by  its  collision  with  the  earth  an 
amount  of  heat  sufficient  to  raise  a  pound  of  water  one 
degree  Fahrenheit  in  temperature.  772  "  foot-pounds  " 
constitute  the  mechanical  equivalent  of  heat.  Now,  a 
body  falling  from  a  height  of  772  feet,  has,  upon 
striking  the  earth,  a  velocity  of  223  feet  a  second ;  and 
if  this  velocity  were  imparted  to  the  body,  by  any  other 


16  FEAGMENTS  OF  SCIEKCE. 

means,  the  quantity  of  heat  generated  by  the  stoppage 
of  its  motion  would  be  that  stated  above.  Six  timea 
that  velocity,  or  1,338  feet,  would  not  be  an  inordina+e 
one  for  a  cannon-ball  as  it  quits  the  gun.  Hence,  a 
canpon-ball  moving  with  a  velocity  of  1,338  feet  a 
second,  would,  by  collision,  generate  an  amount  of  heat 
competent  to  raise  its  own  weight  of  water  36  degrees 
Fahrenheit  in  tern}  erature.  If  composed  of  iron,  and 
if  all  the  heat  generated  were  concentrated  in  the  ball 
itself,  its  temperature  would  be  raised  about  360  degrees 
Fahrenheit ;  because  one  degree  in  the  case  of  water  is 
equivident  to  about  ten  degrees  in  the  case  of  iron.  In 
artillery  practice,  the  heat  generated  is  usually  concen- 
trated upon  the  front  of  the  bolt,  and  on  the  portion  of 
the  target  first  struck.  By  this  concentration  the  heat 
developed  becomes  sufficiently  intense  to  raise  the  dust 
of  the  metal  to  incandescence,  a  flash  of  light  often 
accompanying  collision  with  the  target. 

Let  us  now  fix  our  attention  for  a  moment  on  the 
gimpowder  which  urges  the  cannon-ball.  This  is  com- 
posed of  combustible  matter,  which  if  burnt  in  the  open 
air  would  yield  a  certain  amount  of  heat.  It  will  not 
yield  this  amount  if  it  perform  the  work  of  m-ging  a 
ball.  The  heat  then  generated  by  the  gunpowder  will 
fall  short  of  that  produced  in  the  open  air,  by  an  amount 
equivalent  to  the  vis  viva  of  the  ball ;  and  this  exact 
amount  is  restored  by  the  ball  on  its  collision  with  the 
target.  In  this  perfect  way  are  heat  and  mechanical 
motion  connected. 

Broadly  enunciated,  the  principle  of  the  conservation 
of  force  asserts,  that  the  quantity  of  force  in  the  uni- 
verse is  as  unalterable  as  the  quantity  of  matter ;  that 
it  is  alike  impossible  to  create  force  and  to  annihilate  it. 
But  in  what  sense  are  we  to  understand  this  assertion  ? 
It  would  be  manifestly  inapplicable  to  the  force  of  gravity 


THE  CONSTITUTION   OF  NATURE.  17 

as  defined  by  Newton ;  for  this  is  a  force  varying  inversely 
as  the  square  of  the  distance ;  and  to  affirm  the  con- 
stancy of  a  varying  force  would  be  self-contradictory. 
Yet,  when  the  question  is  properly  understood,  gravity 
forms  no  exception  to  the  law  of  conservation.  Follow- 
ing the  method  pursued  by  Helmholtz,  I  will  here  at- 
tempt an  elementary  exposition  of  this  law.  Though 
destined  in  its  applications  to  produce  momentous 
changes  in  human  thought,  it  is  not  difficult  of  compre- 
hension. 

For  the  sake  of  simplicity  we  will  consider  a  particle 
of  matter,  which  we  may  call  F,  to  be  perfectly  fixed, 
and  a  second  movable  particle,  d,  placed  at  a  distance 
from  F.  We  will  assume  that  these  two  particles  attract 
each  other  according  to  the  Newtonian  law.  At  a  certain 
distance,  the  attraction  is  of  a  certain  definite  amount, 
which  might  be  determined  by  means  of  a  spring  balance. 
At  half  this  distance  the  attraction  would  be  augmented 
four  times  ;  at  a  third  of  the  distance,  nine  times  ;  at 
one-fourth  of  the  distance,  sixteen  times,  and  so^on.  In 
every  case,  the  attraction  might  be  measured  by  deter- 
mining, with  the  spring  balance,  the  amount  of  tension 
just  sufficient  to  prevent  d  from  moving  towards  f. 
Thus  far  we  have  nothing  whatever  to  do  with  motion ; 
we  deal  with  statics,  not  with  dynamics.  We  simply 
take  into  account  the  distance  of  d  from  f,  and  the 
pull  exerted  by  gi-avity  at  that  distance. 

It  is  customary  in  mechanics  to  represent  the  magni- 
tude of  a  force  by  a  line  of  a  certain  length,  a  force  of 
double  magnitude  being  represented  by  a  line  of  double 
length,  and  so  on.  Placing  then  the  particle  d  at  a  dis- 
tance from  f,  we  can,  in  imagination,  draw  a  straight 
line  from  d  to  f,  and  at  d  erect  a  perpendicular  to  this 
line,  which  shall  represent  the  amount  of  the  attraction 
exerted  on  d.   If  d  be  at  a  very  great  distance  from  f,  the 


18  FRAGMENTS  OF  SCIENCE. 

attraction  will  be  very  small,  and  the  perpendicular  conse- 
quently very  short.  If  the  distance  be  practically  infinite, 
the  attraction  is  practically  nil.  Let  us  now  suppose  at 
every  point  in  the  line  joining  f  and  d  a  perpendicular  to 
be  erected,  proportional  in  length  to  the  attraction  exerted 
at  that  point ;  we  thus  obtain  an  infinite  number  of 
perpendiculars,  of  gradually  increasing  length,  as  d  ap- 
proaches F.  Uniting  the  ends  of  all  these  perpendiculars, 
we  obtain  a  cm-ve,  and  between  this  curve  and  the  straight 
line  joining  f  and  d  we  have  an  area  containing  all  the 
perpendiculars  placed  side  by  side.  Each  one  of  this 
infinite  series  of  perpendiculars  representing  an  attrac- 
tion, or  tension,  as  it  is  sometimes  called,  the  area  just 
referred  to  represents  the  sum  of  the  tensions  exerted 
upon  the  particle  d,  during  its  passage  from  its  first 
position  to  f. 

Up  to  the  present  point  we  have  been  dealing  with 
tensions,  not  with  motion.  Thus  far  vis  viva  has  been 
entirely  foreign  to  our  contemplation  of  d  and  f.  Let  us 
now  suppose  i)  placed  at  a  practically  infinite  distance 
from  F ;  here,  as  stated,  the  pull  of  gravity  would  be 
infinitely  small,  and  the  perpendicular  representing  it 
would  dwindle  almost  to  a  point.  In  this  position  the 
sum  of  the  tensions  capable  of  being  exerted  on  d  would 
be  a  maximum.  Let  d  now  begin  to  move  in  obedience 
to  the  infinitesimal  attraction  exerted  upon  it.  Motion 
being  once  set  up,  the  idea  of  vis  viva  arises.  In  moving 
towards  F  the  particle  d  consumes,  as  it  were,  the 
tensions.  Let  us  fix  om:  attention  on  n,  at  any  point  of 
the  path  over  which  it  is  moving.  Between  that  point 
and  F  there  is  a  quantity  of  unused  tensions ;  beyond 
that  point  the  tensions  have  been  all  consumed,  but 
we  have  in  their  place  an  equivalent  quantity  of  vis 
viva.  After  d  has  passed  any  point,  the  tension  pre- 
viously in  store  at  that  point  disappears,  but  not  with- 


THE  CONSTITUTION  OF  NATURE.  19 

out  having  added,  during  the  infinitely  small  duration 
of  its  action,  a  due  amount  of  motion  to  that  previously 
possessed  by  d.  The  nearer  d  approaches  to  f,  the 
smaller  is  the  sum  of  the  tensions  remaining,  but  the 
greater  is  the  vis  viva ;  the  farther  d  is  from  f,  the 
greater  is  the  sum  of  the  un consumed  tensions,  and 
the  less  is  the  living  force.  Now  the  principle  of  con- 
servation affirms  not  the  constancy  of  the  value  of  the 
tensions  of  gravity,  nor  yet  the  constancy  of  the  via 
viva,  taken  separately,  but  the  absolute  constancy  of 
the  value  of  both  taken  together.  At  the  beginning 
the  vis  viva  was  zero,  and  the  tension  area  was  a  maxi- 
mum ;  close  to  F  the  vis  viva  is  a  maximum,  while  the 
tension  area  is  zero.  At  every  other  point,  the  work- 
producing  power  of  the  particle  d  consists  in  part  of 
vis  viva,  and  in  pai't  of  tensions. 

If  gravity,  instead  of  being  attraction,  were  repulsion, 
then,  with  the  particles  in  contact,  the  sum  of  the  tensions 
between  d  and  f  would  be  a  maximum,  and  the  vis  viva 
zero.  If,  in  obedience  to  the  repulsion,  d  moved  away 
from  f,  vis  viva  would  be  generated  ;  and  the  farther  d 
retreated  from  f  the  greater  would  be  its  vis  viva,  and 
the  less  the  amount  of  tension  still  available  for  producing 
motion.  Taking  repulsion  as  well  as  attraction  into 
account,  the  principle  of  the  conservation  of  force  affirms 
that  the  mechanical  value  of  the  tensions  and  vires 
vivoe  of  the  material  universe,  so  far  as  we  know  it,  is 
a  constant  quantity.  The  universe,  in  short,  possesses 
two  kinds  of  property  which  are  mutually  convertible. 
The  diminution  of  either  carries  with  it  the  enhance- 
ment of  the  other,  the  total  value  of  the  property 
remaining  unchanged. 

The  considerations  here  applied  to  gravity  apply 
equally  to  chemical  affinity.  In  a  mixture  of  oxygen  and 
hydrogen  the  atoms  exist  apart,  but  by  the  application 


20  FRAGMENTS  OF  SCIENCE. 

of  proper  means  they  may  be  caused  to  rush  together 
across  that  space  that  separates  them.  While  this 
space  exists,  aod  as  long  as  the  atoms  have  not  begun 
to  move  towards  each  other,  we  have  tensions  and 
nothing  else.  Daring  their  motion  towards  each 
other  the  tensions,  as  in  the  case  of  gravity,  are  con- 
verted into  vis  viva.  After  they  clash  we  have  still 
vis  viva,  but  in  another  form.  It  was  translation,  it 
is  vibration.     It  was  molecular  transfer,  it  is  heat. 

It  is  possible  to  reverse  these  processes,  to  unlock 
the  combined  atoms  and  replace  them  in  their  first 
positions.  But,  to  accomplish  this,  as  much  heat  would 
be  required  as  was  generated  by  their  union.  Such  re- 
versals occur  daily  and  hourly  in  nature.  By  the  solar 
waves,  the  oxygen  of  water  is  divorced  from  its  hydrogen 
in  the  leaves  of  plants.  As  molecular  vis  viva  the 
waves  disappear,  but  in  so  doing  they  re-endow  the 
atoms  of  oxygen  and  hydrogen  with  tension.  The 
atoms  are  thus  enabled  to  recombine,  and  when  they 
do  so  they  restore  the  precise  amount  of  heat  consumed 
in  their  separation.  The  same  remarks  apply  to  the 
compound  of  carbon  and  oxygen,  called  carbonic  acid, 
which  is  exhaled  from  our  lungs,  produced  by  our  fires, 
and  found  sparingly  diffused  everywhere  throughout 
the  air.  In  the  leaves  of  plants  the  sunbeams  also 
wrench  the  atoms  of  carbonic  acid  asunder,  and  sacrifice 
themselves  in  the  act ;  but  when  the  plants  are  burnt, 
the  amount  of  heat  consumed  in  their  production  is 
restored. 

This,  then,  is  the  rhythmic  play  of  Nature  as  regards 
her  forces.  Throughout  all  her  regions  she  oscillates  from 
tension  to  vis  viva,  from  vis  viva  to  tension.  We  have 
the  same  play  in  the  planetary  system.  The  earth's  orbit 
is  an  ellipse,  one  of  the  foci  of  which  is  occupied  by 
the  sun.     Imagine  the  earth  at  the  most  distant  part 


THE  CONSTITUTION  OF  NATURE.  21 

of  the  orbit.  Her  motion,  and  consequently  her  vis 
viva,  is  then  a  minimum.  Tlie  planet  rounds  the 
curve,  and  begins  its  approach  to  the  sun.  In  front  it 
has  a  store  of  tensions,  which  are  gradually  consumed, 
an  e(iuivalent  amount  of  vis  viva  being  generated. 
When  nearest  to  the  sun  the  motion,  and  consequently 
the  vis  viva,  reach  a  maximum.  But  here  the  available 
tensions  have  been  used  up.  The  earth  rounds  this 
portion  of  the  curve  and  retreats  from  the  sun.  Tensions 
are  now  stored  up,  but  vis  viva  is  lost,  to  be  again 
restored  at  the  expense  of  the  complementary  force  on 
the  opposite  side  of  the  curve.  Thus  beats  the  heart 
of  the  universe,  but  without  increase  or  diminution  of 
its  total  stock  of  force. 

I  have  thus  far  tried  to  steer  clear  amid  confusion, 
by  fixing  the  mind  of  the  reader  upon  things  rather 
than  upon  names.  But  good  names  are  essential ;  and 
here,  as  yet,  we  are  not  provided  with  such.  We  have 
had  the  force  of  gravity  and  living  force — two  utterly 
distinct  things.  We  have  had  pulls  and  tensions ; 
and  we  might  have  had  the  force  of  heat,  the  force  of 
light,  the  force  of  magnetism,  or  the  force  of  electricity 
— all  of  which  terms  have  been  employed  more  or  less 
loosely  by  writers  on  physics.  This  confusion  is  happily 
avoided  by  the  introduction  of  the  term  '  energy,'  which 
embraces  both  tension  and  vis  viva.  Energy  is  pos- 
sessed by  bodies  already  in  motion  ;  it  is  then  actual, 
and  we  agree  to  call  it  actual  or  dynamic  energy.  It 
is  our  old  vis  viva.  On  the  other  hand,  energy  is 
possible  to  bodies  not  in  motion,  but  which,  in  virtue 
of  attraction  or  repulsion,  possess  a  power  of  motion 
which  would  realise  itself  if  all  hindrances  were  re- 
moved. Looking,  for  example,  at  gravity ;  a  body  on 
the  earth's  surface  in  a  position  from  which  it  cannot 
fall  to  a  lower  one  possesses  no  energy.     It  has  neither 


22  FEAGMENTS  OF  SCIENCE. 

motion  nor  power  of  motion.  But  the  same  body  sus- 
pended at  a  height  above  the  earth  has  a  power  of  motion, 
though  it  may  not  have  exercised  it.  Energy  is  possible 
to  such  a  body,  and  we  agree  to  call  this  potential 
energy.  It  consists  of  our  old  tensions.  We,  more- 
over, speak  of  the  conservation  of  energy,  instead  of 
the  conservation  of  force  ;  and  say  that  the  sum  of  the 
potential  and  dynamic  energies  of  the  material  universe 
is  a  constant  quantity. 

A  body  cast  upwards  consumes  the  actual  energy  of 
projection,  and  lays  up  potential  energy.  When  it 
reaches  its  utmost  height  all  its  actual  energy  is  con- 
sumed, its  potential  energy  being  then  a  maximum. 
When  it  returns,  there  is  a  reconversion  of  the  poten- 
tial into  the  actual.  A  pendulum  at  the  limit  of  its 
swing  possesses  potential  energy ;  at  the  lowest  point 
of  its  arc  its  energy  is  all  actual.  A  patch  of  snow 
resting  on  a  mountain  slope  has  potential  energy; 
loosened,  and  shooting  down  as  an  avalanche,  it  pos- 
sesses dynamic  energy.  The  pine-trees  growing  on  the 
Alps  have  potential  energy ;  but  rushing  down  the 
Holzrinne  of  the  woodcutters  they  possess  actual 
energy.  The  same  is  true  of  the  mountains  themselves. 
As  long  as  the  rocks  which  compose  them  can  fall  to  a 
lower  level,  they  possess  potential  energy,  which  is 
converted  into  actual  when  the  frost  ruptures  their 
cohesion  and  hands  them  over  to  the  action  of  gravity. 
The  stone  avalanches  of  the  Matterhorn  and  Weisshorn 
are  illustrations  in  point.  The  hammer  of  the  great 
bell  of  Westminster,  when  raised  before  striking,  pos- 
sesses potential  energy ;  when  it  falls,  the  energy 
becomes  dynamic ;  and  after  the  stroke,  we  have  tne 
rhythmic  play  of  potential  and  dynamic  in  the  vibra- 
tions of  the  bell.  The  same  holds  good  for  the  molecular 
oscillations  of    a  heated    body.      An   atom  is  driveD 


THE  CONSTITUTION  OF  NATURE.  23 

against  its  neighbour,  and  recoils.  The  ultimate 
amplitude  of  the  recoil  being  attained,  the  motion  of 
the  atom  in  that  direction  is  checked,  and  for  an 
instant  its  energy  is  all  potential.  It  is  then  drawn 
towards  its  neighbour  with  accelerated  speed  ;  thus,  by 
attraction,  converting  its  potential  into  dynamic 
energy.  Its  motion  in  this  direction  is  also  finally 
checked,  and  again,  for  an  instant,  its  energy  is  all 
potential.  It  once  more  retreats,  converting,  by  re- 
pulsion, its  potential  into  dynamic  energy,  till  the 
latter  attains  a  maximum,  after  which  it  is  again 
changed  into  potential  energy.  Thus,  what  is  true  of 
the  earth,  as  she  swings  to  and  fro  in  her  yearly  journey 
round  the  sun,  is  also  true  of  her  minutest  atom.  We 
have  wheels  within  wheels,  and  rhythm  within  rhythm. 

When  a  body  is  heated,  a  change  of  molecular 
arrangement  always  occurs,  and  to  produce  this  change 
heat  is  consumed.  Hence,  a  portion  only  of  the  heat 
communicated  to  the  body  remains  as  dynamic  energy. 
Looking  back  on  some  of  the  statements  made  at  the 
beginning  of  this  article,  now  that  our  knowledge  is 
more  extensive,  we  see  the  necessity  of  qualifying  them» 
When,  for  example,  two  bodies  clash,  heat  is  generated  ; 
but  the  heat,  or  molecular  dynamic  energy,  developed 
at  the  moment  of  collision,  is  not  the  exact  equivalent 
of  the  sensible  dynamic  energy  destroyed.  The  true 
equivalent  is  this  heat,  plus  the  potential  energy  con- 
ferred upon  the  molecules  by  the  placing  of  greater- 
distances  between  them.  This  molecular  potential 
energy  is  afterwards,  on  the  cooling  of  the  body,  con- 
verted into  heat. 

Wherever  two  atoms  capable  of  uniting  together  by 

their  mutual  attractions  exist  separately,  they  form  a 

store  of  potential  energy.     Thus  our  woods,  forests,  and 

coal-fields  on  the  one  hand,  and  our  atmospheric  oxygen 

8 


24  FEAaMENTS  OF  SCIENCE. 

on  the  other,  constitute  a  vast  store  of  energy  of  this 
kind — vast,  but  far  from  infinite.  We  have,  besides 
our  coal-fields,  metallic  bodies  more  or  less  sparsely  dis- 
tributed through  the  earth's  crust.  These  bodies  can 
be  oxydised ;  and  hence  they  are,  so  far  as  they  go,  stores 
of  energy.  But  the  attractions  of  the  great  mass  of  the 
earth's  crust  axe  already  satisfied,  and  from  them  no 
further  energy  can  possibly  be  obtained.  Ages  ago  the 
elementary  constituents  of  our  rocks  clashed  together 
and  produced  the  motion  of  heat,  which  was  taken  up 
by  the  ether  and  carried  away  through  stellar  space. 
It  is  lost  for  ever  as  far  as  we  are  concerned.  In  those 
ages  the  hot  conflict  of  carbon,  oxygen,  and  calcium 
produced  the  chalk  and  limestone  hills  which  are  now 
cold ;  and  from  this  carbon,  oxygen,  and  calcium  no 
further  energy  can  be  derived.  So  it  is  with  almost  all 
the  other  constituents  of  the  earth's  crust.  They  took 
their  present  form  in  obedience  to  molecular  force ;  they 
tinned  their  potential  energy  into  dynamic,  and  yielded 
it  as  radiant  heat  to  the  universe,  ages  before  man  ap- 
peared upon  this  planet.  For  him  a  residue  of  potential 
energy  remains,  vast,  truly,  in  relation  to  the  life  and 
wants  of  an  individual,  but  exceedingly  minute  in  com- 
parison with  the  earth's  primitive  store. 

To  sum  up.  The  whole  stock  of  energy  or  working- 
power  in  the  world  consists  of  attractions,  repulsions, 
and  motions.  If  the  attractions  and  repulsions  be  so 
circumstanced  as  to  be  able  to  produce  motion,  they  are 
sources  of  working-power,  but  not  otherwise.  As  stated 
a  moment  ago,  the  attraction  exerted  between  the  earth 
and  a  body  at  a  distance  from  the  earth's  smface,  is  a 
source  of  working-power;  because  the  body  can  be  moved 
by  the  attraction,  and  in  falling  can  perform  work. 
When  it  rests  at  its  lowest  level  it  is  not  a  source  of 
power  or  energy,  because  it  can  fall  no  farther.     But 


THE  CONSTITUTION   OF  NATURE.  26 

though  it  has  ceased  to  be  a  source  of  energy,  the  at- 
traction of  gravity  still  acts  as  a  force,  which  holds  the 
earth  and  weight  together. 

The  same  remarks  apply  to  attracting  atoms  and 
molecules.  As  long  as  distance  separates  them,  they 
can  move  across  it  in  obedience  to  the  attraction  ;  and 
the  motion  thus  produced  may,  by  proper  appliances, 
be  caused  to  perform  mechanical  work.  When,  for  ex- 
ample, two  atoms  of  hydrogen  unite  with  one  of  oxygen, 
to  form  water,  the  atoms  are  first  drawn  towards  each 
otlier — they  move,  they  clash,  and  then  by  virtue  of 
their  resiliency,  they  recoil  and  quiver.  To  this  quiver- 
ing motion  we  give  the  name  of  heat.  This  atomic 
vibration  is  merely  the  redistribution  of  the  motion 
produced  by  the  chemical  affinity ;  and  this  is  the  only 
sense  in  which  chemical  affinity  can  be  said  to  be  con- 
verted into  heat.  We  must  not  imagine  the  chemical 
attraction  destroyed,  or  converted  into  anything  else. 
For  the  atoms,  when  mutually  clasped  to  form  a  mole- 
cule of  water,  are  held  together  by  the  very  attraction 
which  first  drew  them  towards  each  other.  That  which 
has  really  been  expended  is  the  jpull  exerted  through 
the  space  by  which  the  distance  between  the  atoms  has 
been  diminished. 

If  this  be  understood,  it  will  be  at  once  seen  that 
gravity,  as  before  insisted  on,  may,  in  this  sense,  be  said 
to  be  convertible  into  heat ;  that  it  is  in  reality  no  more 
an  outstanding  and  inconvertible  agent,  as  it  is  some- 
times stated  to  be,  than  is  chemical  affinity.  By  the 
exertion  of  a  certain  pull  through  a  certain  space,  a 
body  is  caused  to  clash  with  a  certain  definite  velocity 
againfit  the  earth.  Heat  is  thereby  developed,  and  this 
is  the  only  sense  in  which  gravity  can  be  said  to  be  con- 
verted into  heat.  In  no  case  is  the  force  which  produces 
the  motion  annihilated  or  changed  into  anything  else. 


26  FRAGMENTS  OF  SCIENCE. 

The  mutual  attraction  of  the  earth  and  weight  exists 
when  they  are  in  contact,  as  when  they  were  separate  ; 
but  the  ability  of  that  attraction  to  employ  itself  in  the 
production  of  motion  does  not  exist. 

The  transformation,  in  this  case,  is  easily  followed 
by  the  mind's  eye.  First,  the  weight  as  a  whole  is  set 
in  motion  by  the  attraction  of  gravity.  This  motion  of 
the  mass  is  arrested  by  collision  with  the  earth,  being 
broken  up  into  molecular  tremors,  to  which  we  give  the 
name  of  heat. 

And  when  we  reverse  the  process,  and  employ  those 
tremors  of  heat  to  raise  a  weight — which  is  done  through 
the  intermediation  of  an  elastic  fluid  in  the  steam-engine 
— a  certain  definite  portion  of  the  molecular  motion  is 
consumed.  In  this  sense,  and  in  this  sense  only,  can 
the  heat  be  said  to  be  converted  into  gravity ;  or,  more 
correctly,  into  potential  energy  of  gravity.  Here  the 
destruction  of  the  heat  has  created  no  new  attraction  ; 
but  the  old  attraction  has  conferred  upon  it  a  power  of 
exerting  a  certain  definite  pull,  between  the  starting- 
point  of  the  falling  weight  and  the  earth. 

When,  therefore,  writers  on  the  conservation  of 
energy  speak  of  tensions  being  '  consumed '  and  '  gene- 
rated,' they  do  not  mean  thereby  that  old  attractioris 
have  been  annihilated,  and  new  ones  brought  into  exist- 
ence, but  that,  in  the  one  case,  the  power  of  the  attrac- 
tion to  produce  motion  has  been  diminished  by  the 
shortening  of  the  distance  between  the  attracting  bodies, 
while,  in  the  other  case,  the  power  of  producing  motion 
has  been  augmented  by  the  increase  of  the  distance. 
These  remarks  apply  to  all  bodies,  whether  they  be  sen- 
sible masses  or  molecules. 

Of  the  inner  quality  that  enables  matter  to  attract 
matter  we  know  nothing ;  and  the  law  of  conservation 
makes  no  statement  regarding  that  quality.     It  takes 


THE  CONSTITUTION  OF  NATUEE.  37 

the  facts  of  attraction  as  they  stand,  and  affirms  only 
the  constancy  of  working-power.  That  power  may  exist 
in  the  form  of  motion  ;  or  it  may  exist  in  the  form  of 
FORCE,  with  distance  to  act  through.  The  former  is 
dynamic  energy,  the  latter  is  potential  energy,  the  con- 
staiicy  of  the  sum  of  both  being  affirmed  by  the  law  of 
conservation.  The  convertibility  of  natural  forces  con- 
sists solely  in  transformations  of  dynamic  into  potential, 
and  of  potential  into  dynamic  energy.  In  no  other  sense 
has  the  convertibility  of  force  any  scientific  meaning. 


Grave  errors  have  been  entertained  as  to  what  is 
really  intended  to  be  conserved  by  the  doctrine  of  con- 
servation. This  exposition  I  hope  will  tend  to  remove 
them. 


28  FRAGMENTS  OF  SQIENCK. 


IL 

BABIATIOm 

1.  VisihLe  and  Invisible  Radiation, 

BETWEEN  the  mind  of  man  and  the  outer  world  are 
interposed  the  nerves  of  the  human  body,  which 
translate,  or  enable  the  mind  to  translate,  the  im- 
pressions of  that  world  into  facts  of  consciousness  and 
thought. 

Different  nerves  are  suited  to  the  perception  of 
different  impressions.  We  do  not  see  with  the  ear,  nor 
hear  with  the  eye,  nor  are  we  rendered  sensible  of  sound 
by  the  nerves  of  the  tongue.  Out  of  the  general  assem- 
blage of  physical  actions,  each  nerve,  or  group  of  nerves, 
selects  and  responds  to  those  for  the  perception  of  which 
it  is  specially  organised. 

The  optic  nerve  passes  from  the  brain  to  the  back 
of  the  eyeball  and  there  spreads  out,  to  form  the  retina, 
a  web  of  nerve  filaments,  on  which  the  images  of  ex- 
ternal objects  are  projected  by  the  optical  portion  of 
the  eye.  This  nerve  is  limited  to  the  apprehension  of 
the  phenomena  of  radiation,  and,  notwithstanding  its 
marvellous  sensibility  to  certain  impressions  of  this  class, 
it  is  singularly  obtuse  to  other  impressions. 

'  The  Rede  LeQture  delivered  in  the  Senate  House  before  the 
University  of  Cambridge,  May  16,  1865. 


RADIATION.  29 

Nor  does  the  optic  nerve  embrace  the  entire  range 
even  of  radiation.  Some  rays,  when  they  reach  it,  are 
incompetent  to  evoke  its  power,  while  others  never 
reach  it  at  all,  being  absorbed  by  the  humours  of  the 
eye.  To  all  rays  which,  whether  they  reach  the  retina 
or  not,  fail  to  excite  vision,  we  give  the  name  of  invisible 
or  obscure  rays.  All  non-luminous  bodies  emit  such 
rays.  There  is  no  body  in  nature  absolutely  cold,  and 
every  body  not  absolutely  cold  emits  rays  of  heat.  But 
to  render  radiant  heat  fit  to  affect  the  optic  nerve  a 
certain  temperature  is  necessary.  A  cool  poker  thrust 
into  a  fire  remains  dark  for  a  time,  but  when  its  tem- 
perature has  become  equal  to  that  of  the  surrounding 
coals,  it  glows  like  them.  In  like  manner,  if  a  current 
of  electricity,  of  gradually  increasing  strength,  be  sent 
through  a  wire  of  the  refractory  metal  platinum,  the 
wire  first  becomes  sensibly  warm  to  the  touch;  for  a 
time  its  heat  augments,  still  however  remaining  obscure ; 
at  length  we  can  no  longer  touch  the  metal  with  im- 
punity ;  and  at  a  certain  definite  temperature  it  emits 
a  feeble  red  light.  As  the  current  augments  in  power 
the  light  augments  in  brilliancy,  until  finally  the  wire 
appears  of  a  dazzling  white.  The  light  which  it  now 
emits  is  similar  to  that  of  the  sun. 

By  means  of  a  prism  Sir  Isaac  Newton  unravelled 
the  texture  of  solar  light,  and  by  the  same  simple  instru- 
ment we  can  investigate  the  luminous  changes  of  our 
platinum  wire.  In  passing  through  the  prism  all  its 
rays  (and  they  are  infinite  in  variety)  are  bent  or  re- 
fracted from  their  straight  course ;  and,  as  different  rays 
are  differently  refracted  by  the  prism,  we  are  by  it  en- 
abled to  separate  one  class  of  rays  from  another.  By 
such  prismatic  analysis  Dr.  Draper  has  shown,  that  when 
the  platinum  wire  first  begins  to  glow,  the  light  emitted 
is  sensibly  red.    As  the  glow  augments  the  red  becomes 


80  FRAGMENTS  OF  SCIENCE. 

more  brilliant,  but  at  the  same  time  orange  lays  are 
added  to  the  emission.  Augmenting  the  temperature 
still  further,  yellow  rays  appear  beside  the  orange;  after 
the  yellow,  green  rays  are  emitted  ;  and  after  the  green 
come,  in  succession,  blue,  indigo,  and  violet  rays.  To 
display  all  these  colours  at  the  same  time  the  platinum 
wire  must  be  white-hot :  the  impression  of  whiteness 
being  in  fact  produced  by  the  simultaneous  action  of  all 
these  colours  on  the  optic  nerve. 

In  the  experiment  just  described  we  began  with  a 
platinum  wire  at  an  ordinary  temperature,  and  gradually 
raised  it  to  a  white  heat.  At  the  beginning,  and  even 
before  the  electric  ciu'rent  had  acted  at  all  upon  the 
wire,  it  emitted  invisible  rays.  For  some  time  after 
the  action  of  the  current  had  commenced,  and  even  for 
a  time  after  the  wire  had  become  intolerable  to  tlie 
touch,  its  radiation  was  still  invisible.  The  question 
now  arises.  What  becomes  of  these  invisible  rays  when 
the  visible  ones  make  their  appearance  ?  It  will  be 
proved  in  the  sequel  that  they  maintain  themselves  in 
the  radiation ;  that  a  ray  once  emitted  continues  to  be 
emitted  when  the  temperature  is  increased,  and  hence 
the  emission  from  our  platinum  wire,  even  when  it  has 
attained  its  maximum  brilliancy,  consists  of  a  mixture 
of  visible  and  invisible  rays.  If,  instead  of  the  platinum 
wire,  the  earth  itself  were  raised  to  incandescence,  the 
obscure  radiation  which  it  now  emits  would  continue  to 
be  emitted.  To  reach  incandescence  the  planet  would 
have  to  pass  through  all  the  stages  of  non-luminous 
radiation,  and  the  final  emission  would  embrace  the  rays 
of  all  these  stages.  There  can  hardly  be  a  doubt  that 
from  the  sun  itself,  rays  proceed  similar  in  kind  to  those 
which  the  dark  earth  pours  nightly  into  space.  In  fact, 
the  various  kind  of  obscure  rays  emitted  by  all  the 


RADIATION.  81 

planets  of  our  system  are  included  in  the  present  radia- 
tion of  the  sun. 

The  great  pioneer  in  this  domain  of  science  was  Sir 
William  Herschel.  Causing  a  beam  of  solar  light  to 
pass  through  a  prism,  he  resolved  it  into  its  coloured 
constituents  ;  he  formed  what  is  technically  called  the 
solar  spectrum.  Exposing  thermometers  to  the  suc- 
cessive colours  he  determined  their  heating  power,  and 
found  it  to  augment  from  the  violet  or  most  refracted 
end,  to  the  red  or  least  refracted  end  of  the  spectrum. 
But  he  did  not  stop  here.  Pushing  his  thermometers 
into  the  dark  space  beyond  the  red  he  found  that,  though 
the  light  had  disappeared,  the  radiant  heat  falling  on 
the  instruments  was  more  intense  than  that  at  any 
visible  part  of  the  spectrum.  In  fact.  Sir  William 
Herschel  showed,  and  his  results  have  been  verified  by 
various  philosophers  since  4iis  time,  that,  besides  its 
luminous  rays,  the  sun  pours  forth  a  multitude  of  other 
rays,  more  powerfully  calorific  than  the  luminous  ones, 
but  entirely  unsuited  to  the  purposes  of  vision. 

At  the  less  refrangible  end  of  the  solar  spectrum, 
then,  the  range  of  the  sun's  radiation  is  not  limited  by 
that  of  the  eye.  The  same  statement  applies  to  the 
more  refrangible  end.  Eitter  discovered  tne  extension 
of  the  spectrum  into  the  invisible  region  beyond  the 
violet ;  and,  in  recent  times,  this  ultra-violet  emission 
has  had  peculiar  interest  conferred  upon  it  by  the  ad- 
nairable  researches  of  Professor  Stokes.  The  complete 
spectrum  of  the  sun  consists,  therefore,  of  three  distinct 
parts  : — first,  of  ultra-red  rays  of  high  heating  power, 
but  unsuited  to  the  purposes  of  vision ;  secondly,  of 
luminous  rays  which  display  the  succession  of  colours, 
red,  orange,  yellow,  green,  blue,  indigo,  violet ;  thirdly, 
of  ultra-violet  rays  which,  like  the  ultra-red  ones,  are 


32  FRAGMENTS  OF  SCIENCE. 

incompetent  to  excite  vision,  but  which,  unlike  the 
ultra-red  rays,  possess  a  very  feeble  heating  power.  In 
consequence,  however,  of  their  chemical  energy  these 
ultra-violet  rays  are  of  the  utmost  importance  to  the 
organic  world. 

2.  Origm  cmd  Cha/rader  of  Radiation,     The  Ether. 

When  we  see  a  platinum  wire  raised  gradually  to  a 
white  heat,  and  emitting  in  succession  all  the  colours  of 
the  spectrum,  we  are  simply  conscious  of  a  series  of 
changes  in  the  condition  of  our  own  eyes.  We  do  not 
see  the  actions  in  which  these  successive  colours  origin- 
ate, but  the  mind  irresistibly  infers  that  the  appearance 
of  the  colours  corresponds  to  certain  contemporaneous 
changes  in  the  wire.  What  is  the  nature  of  these 
changes  ?  In  virtue  of  what  condition  does  the  wire 
radiate  at  all  ?  We  must  now  look  from  the  wire,  as 
a  whole,  to  its  constituent  atoms.  Could  we  see  those 
atoms,  even  before  the  electric  current  has  begun  to 
act  upon  them,  we  should  find  them  in  a  state  of  vibra- 
tion. In  this  vibration,  indeed,  consists  such  warmth 
as  the  wire  then  possesses.  Locke  enunciated  this  idea 
with  great  precision,  and  it  has  been  placed  beyond  the 
pale  of  doubt  by  the  excellent  quantitative  researches  of 
Mr.  Joule.  '  Heat,'  says  Locke,  '  is  a  very  brisk  agita- 
tion of  the  insensible  parts  of  the  object,  which  produce 
in  us  that  sensation  from  which  we  denominate  the 
object  hot :  so  what  in  our  sensations  is  heat  in  the  object 
is  nothing  but  motion,''  When  the  electric  current, 
still  feeble,  begins  to  pass  through  the  wire,  its  first 
act  is  to  intensify  the  vibrations  already  existing,  by 
causing  the  atoms  to  swing  through  wider  ranges. 
Technically  speaking,  the  ant'plitudes  of  the  oscillations 
Me  increased.     The  current  does  this,  however,  without 


RADIATION.  3^ 

altering  the  periods  of  the  old  vibrations,  or  the  times 
in  which  they  were  executed.  But  besides  intensifying 
the  old  vibrations  the  current  generates  new  and  more 
rapid  ones,  and  when  a  certain  efinite  rapidity  has  been 
attained,  the  wire  begins  to  glow.  The  colour  first 
exhibited  is  red,  which  corresponds  to  the  lowest  rate 
of  vibration  of  which  the  eye  is  able  to  take  cognisance. 
By  augmenting  the  strength  of  the  electric  current 
more  rapid  vibrations  are  introduced,  and  orange  rays 
appear.  A  quicker  rate  of  vibration  produces  yellow,  a 
still  quicker  green ;  and  by  further  augmenting  the 
rapidity,  we  pass  through  blue,  indigo,  and  violet,  to  the 
extreme  ultra-violet  rays. 

Such  are  the  changes  recognised  by  the  mind  in  the 
wire  itself,  as  concurrent  with  the  visual  changes  taking 
place  in  the  eye.  But  what  connects  the  wire  with  this 
organ  ?  By  what  means  does  it  send  such  intelligence 
of  its  varying  condition  to  the  optic  nerve  ?  Heat  being 
as  defined  by  Locke,  '  a  very  brisk  agitation  of  the  insen- 
sible parts  of  an  object,'  it  is  readily  conceivable  that  on 
touching  a  heated  body  the  agitation  may  communicate 
itself  to  the  adjacent  nerves,  and  announce  itself  to  them 
as  light  or  heat.  But  the  optic  nerve  does  not  touch 
the  hot  platinum,  and  hence  the  pertinence  of  the  ques- 
tion. By  what  agency  are  the  vibrations  of  the  wire 
transmitted  to  the  eye  ? 

The  answer  to  this  question  involves  one  of  the  most 
important  physical  conceptions  that  the  mind  of  man 
has  yet  achieved :  the  conception  of  a  medium  filling 
space  and  fitted  mechanically  for  the  transmission  of 
the  vibrations  of  light  and  heat,  as  air  is  fitted  for  the 
transmission  of  sound.  This  medium  is  called  the 
luminiferous  ether.  Every  vibration  of  every  atom 
of  our  platinum  wire  raises  in  this  ether  a  wave,  which 
speeds  through  it  at  the  rate  of  186,000  miles  a  second- 


34  FEAGMENTS  OF  SCIENCE. 

The  ether  suffers  no  rupture  of  continuity  at  tho 
surface  of  the  eye,  the  inter-molecular  spaces  of  the 
various  humours  are  filled  with  it ;  hence  the  waves 
generated  by  the  glowing  platinum  can  cross  these 
humours  and  impinge  on  the  optic  nerve  at  the  back  of 
the  eye.*  Thus  the  sensation  of  light  reduces  itself  to 
the  acceptance  of  motion.  Up  to  this  point  we  deal 
with  pure  mechanics ;  but  the  subsequent  translation 
of  the  shock  of  the  ethereal  waves  into  consciousness 
eludes  mechanical  science.  As  an  oar  dipping  into 
the  Cam  generates  systems  of  waves,  which,  speeding 
from  the  centre  of  disturbance,  finally  stir  the  sedges 
on  the  river's  bank,  so  do  the  vibrating  atoms  generate 
in  the  surrounding  ether  undulations,  which  finally  stir 
the  filaments  of  the  retina.  The  motion  thus  imparted 
is  transmitted  with  measurable,  and  not  very  great 
velocity  to  the  brain,  where,  by  a  process  which  the 
science  of  mechanics  does  not  even  tend  to  unravel,  the 
tremor  of  the  nervous  matter  is  converted  into  the  con- 
scious impression  of  light. 

Darkness  might  then  be  defined  as  ether  at  rest ; 
light  as  ether  in  motion.  But  in  reality  the  ether  is 
never  at  rest,  for  in  the  absence  of  light-waves  we  have 
heat-waves  always  speeding  through  it.  In  the  spaces 
of  the  universe  both  classes  of  undulations  incessantly 
commingle.  Here  the  waves  issuing  from  uncounted 
centres  cross,  coincide,  oppose,  and  pass  through  each 
other,  without  confusion  or  ultimate  extinction.  Every 
star  is  seen  across  the  entanglement  of  wave-motions 
produced  by  all  other  stars.  It  is  the  ceaseless  thrill 
caused  by  those  distant  orbs  collectively  in  the  ether, 
that  constitutes  what  we  call  the  '  temperature  of  space.' 
As  the  air  of  a  room  accommodates  itself  to  the  require- 

*  The  action  here  described  is  analogous  to  the  passage  of  sound- 
waves through  thick  felt  whose  interstices  are  occupied  by  air. 


RADIATION.  35 ' 

ments  of  an  orchestra,  transmitting  each  vibration  of 
every  pipe  and  string,  so  does  the  inter-stellar  ether 
accommodate  itself  to  the  requirements  of  light  and 
heat.  Its  waves  mingle  in  space  without  disorder, 
each  being  endowed  with  an  individuality  as  inde- 
structible as  if  it  alone  had  disturbed  the  universal 
repose. 

All  vagueness  with  regard  to  the  use  of  the  terms 
*  radiation'  and  'absorption'  will  now  disappear. 
Eadiation  is  the  communication  of  vibratory  motion  to 
the  ether ;  and  when  a  body  is  said  to  be  chilled  by 
radiation,  as  for  example  the  grass  of  a  meadow  on  a 
starlight  night,  the  meaning  is,  that  the  molecules  of 
the  grass  have  lost  a  portion  of  their  motion,  by  im- 
parting it  to  the  medium  in  which  they  vibrate.  On 
the  other  hand,  the  waves  of  ether  may  so  strike 
against  the  molecules  of  a  body  exposed  to  their  action 
as  to  yield  up  their  motion  to  the  latter ;  and  in  this 
transfer  of  the  motion  from  the  ether  to  the  molecules 
consists  the  absorption  of  radiant  heat.  All  the  pheno- 
mena of  heat  are  in  this  way  reducible  to  interchanges 
of  motion ;  and  it  is  purely  as  the  recipients  or  the 
donors  of  this  motion,  that  we  ourselves  become  con- 
scious of  the  action  of  heat  and  cold. 


3.  The  Atomic  Theory  in  reference  to  the  Ether, 

The  word  '  atoms '  has  been  more  than  once  em- 
ployed in  this  discourse.  Chemists  have  taught  us  that 
all  matter  is  reducible  to  certain  elementary  forms  to 
which  they  give  this  name.  These  atoms  are  endowed 
with  powers  of  mutual  attraction,  and  under  suitable 
circumstances  they  coalesce  to  form  compounds.  Thus 
oxygen  and  hydrogen  are  elements  when  separate,  ox 


36  FRAaMENTS  OF  SCIENCE. 

merely  mixed,  but  they  may  be  made  to  comhine  so  as 
to  form  molecules,  each  consisting  of  two  atoms  of 
hydrogen  and  one  of  oxygen.  In  this  condition  they 
constitute  water.  Sp  also  chlorine  and  sodium  are 
elements,  the  former  a  pungent  gas,  the  latter  a  soft 
metal ;  and  they  unite  together  to  form  chloride  of 
sodium  or  common  salt.  In  the  same  way  the  element 
nitrogen  combines  with  hydrogen,  in  the  proportion  of 
one  atom  of  the  former  to  three  of  the  latter,  to  form 
ammonia.  Picturing  in  imagination  the  atoms  of  ele- 
mentary bodies  as  little  spheres,  the  molecules  of  com- 
pound bodies  must  be  pictured  as  groups  of  such  spheres. 
This  is  the  atomic  theory  as  Dalton  conceived  it.  Now 
if  this  theory  have  any  foundation  in  fact,  and  if  the 
theory  of  an  ether  pervading  space,  and  constituting 
the  vehicle  of  atomic  motion,  be  founded  in  fact,  it  is 
surely  of  interest  to  examine  whether  the  vibiations  of 
elementary  bodies  are  modified  by  the  act  of  combina- 
tion— whether  as  regards  radiation  and  absorption,  or, 
in  other  words,  whether  as  regards  the  communication 
of  motion  to  the  ether,  and  the  acceptance  of  motion 
from  it,  the  deportment  of  the  uncombined  atoms  will 
be  different  from  that  of  the  combined. 


4.  Absorption  of  Radiant  Heat  by  Gases, 

We  have  now  to  submit  these  considerations  to  the 
only  test  by  which  they  can  be  tried,  namely,  that  of 
experiment.  An  experiment  is  well  defined  as  a  ques- 
tion put  to  Nature ;  but,  to  avoid  the  risk  of  asking 
amiss,  we  ought  to  purify  the  question  from  all  adjuncts 
which  do  not  necessarily  belong  to  it.  Matter  has 
been  shown  to  be  composed  of  elementary  constituents, 
by  the  compounding  of  which  all  its  varieties  are  pro- 


EADIATION.  37 

duced.  But,  besides  the  chemical  unions  which  they 
form,  both  elementary  and  compound  bodies  can  unite 
in  another  and  less  intimate  way.  Gases  and  vapours 
aggregate  to  liquids  and  solids,  without  any  change  of 
their  chemical  nature.  We  do  not  yet  know  how  the 
transmission  of  radiant  heat  may  be  affected  by  the 
entanglement  due  to  cohesion  ;  and,  as  our  object  now 
is  to  examine  the  influence  of  chemical  union  alone,  we 
shall  render  our  experiments  more  pure  by  liberating 
the  atoms  and  molecules  entirely  from  the  bonds  of  cohe- 
sion, and  employing  them  in  the  gaseous  or  vaporous  form. 

Let  us  endeavour  to  obtain  a  perfectly  clear  mental 
image  of  the  problem  now  before  us.  Limiting  in  the 
first  place  our  enquiries  to  the  phenomena  of  absorp- 
tion, we  have  to  picture  a  succession  of  waves  issuing 
from  a  radiant  source  and  passing  through  a  gas  ;  some 
of  them  striking  against  the  gaseous  molecules  and 
yielding  up  their  motion  to  the  latter ;  others  gliding 
round  the  molecules,  or  passing  through  the  inter- 
molecular  spaces  without  apparent  hindrance.  The 
problem  before  us  is  to  determine  whether  such  free 
molecules  have  any  power  whatever  to  stop  the  waves 
of  heat ;  and  if  so,  whether  different  molecules  possess 
this  power  in  different  degrees. 

In  examining  the  problem  let  us  fall  back  upon  an 
actual  piece  of  work,  choosing  as  the  source  of  our  heat- 
waves a  plate  of  copper,  against  the  back  of  whicli  a 
steady  sheet  of  flame  is  permitted  to  play.  On  emerging 
from  the  copper,  the  waves,  in  the  first  instance,  pass 
through  a  space  devoid  of  air,  and  then  enter  a  hollow 
glass  cylinder,  three  feet  long  and  three  inches  wide. 
The  two  ends  of  this  cylinder  are  stopped  by  two  plates 
of  rock-salt,  a  solid  substance  which  offers  a  scarcely 
sensible  obstacle  to  the  passage  of  the  calorific  waves. 
After  passing  through  the  tube,  the  radiant  heat  falla 


88  FEAQMENTS  OF  SCIENCE. 

upon  the  anterior  face  of  a  thermo-electric  pile,'  which 
instantly  converts  the  heat  into  an  electric  current. 
This  current  conducted  round  a  magnetic  needle  de- 
flects it,  and  the  magnitude  of  the  deflection  is  a 
measure  of  the  heat  falling  upon  the  pile.  This  famous 
instrument,  and  not  an  ordinary  thermometer,  is  what 
we  shall  use  in  these  enquiries,  but  we  shall  use  it  in  a 
somewhat  novel  way.  As  long  as  the  two  opposite 
faces  of  the  thermo-electric  pile  are  kept  at  the  same 
temperature,  no  matter  how  high  that  may  be,  there  is 
no  current  generated.  The  current  is  a  consequence  of 
a  difference  of  temperature  between  the  two  opposite 
faces  of  the  pile.  Hence,  if  after  the  anterior  face  has 
received  the  heat  from  our  radiating  source,  a  second 
source,  which  we  may  call  the  compensating  source,  be 
permitted  to  radiate  against  the  posterior  face,  this 
latter  radiation  will  tend  to  neutralise  the  former. 
When  the  neutralisation  is  perfect,  the  magnetic 
needle  connected  with  the  pile  is  no  longer  deflected, 
but  points  to  the  zero  of  the  graduated  circle  over  which 
it  hangs. 

And  now  let  us  suppose  the  glass  tube,  through 
which  the  waves  from  the  heated  plate  of  copper  are 
passing,  to  be  exhausted  by  an  air-pump,  the  two 
sources  of  heat  acting  at  the  same  time  on  the  two 
opposite  faces  of  the  pile.  When  by  means  of  an  ad- 
justing screen,  perfectly  equal  quantities  of  heat  are 
imparted  to  the  two  faces,  the  needle  points  to  zero. 
Let  any  gas  be  now  permitted  to  enter  the  exhausted 
tube  ;  if  its  molecules  possess  any  power  of  intercepting 
the  calorific  waves,  the  equilibrium  previously  existing 
will  be  destroyed,  the  compensating  source  will  triumph, 

>  In  the  Appendix  to  the  fii-st  chapter  of  *  Heat  as  a  Mode  of 
Motion,'  the  construction  of  the  thermo-electric  pile  is  fully  ex- 
plained. 


RADAITION. 


39 


and  a  deflection  of  the  magnetic  needle  will  be  the 
immediate  consequence.  From  the  deflections  thus 
produced  by  different  gases,  we  can  readily  deduce  the 
relative  amounts  of  wave-motion  which  their  molecules 
intercept. 

In  this  way  the  substances  mentioned  in  the  follow- 
ing table  were  examined,  a  small  portion  only  of  each 
being  admitted  into  the  glass  tube.  The  quantity  ad- 
mittjfed  in  each  case  was  just  sufficient  to  depress  a  column 
of  mercury  associated  with  the  tube  one  inch  :  in  other 
words,  the  gases  were  examined  at  a  pressure  of  one- 
thirtieth  of  an  atmosphere.  The  numbers  in  the  table 
express  the  relative  amounts  of  wave-motion  absorbed 
by  the  respective  gases,  the  quantity  intercepted  bj 
atmospheric  air  being  taken  as  unity. 

Radiation  through  Gases, 


Relative 

Name  of  gus                                           absorption 

Air                 1 

Oxygen. 

1 

Nitrogen        , 

I 

Hydrogen       .        , 

1 

Carbonic  oxide      • 

.      750 

Carbonic  acid 

.      973 

Hydrochloric  acid . 

1,005 

Nitric  oxide  . 

1,590 

Nitrous  oxide 

1,860 

Sulphide  of  hydrogen    . 

2,100 

Ammonia       .        .        , 

5,460 

defiant  gas   .        ,        , 

6,030 

Sulphurous  acid     •        , 

6,480 

Every  gas  in  this  table  is  perfectly  transparent  to 
light,  that  is  to  say,  all  waves  within  the  limits  of  the 
visible  spectrum  pass  through  it  without  obstruction  ; 
but  for  the  waves  of  slower  period,  emanating  from  our 
heated  plate  of  copper,  enormous  differences  of  absorp- 
tive power  are  manifested.  These  differences  illustrate 
4 


40  FRAGMENTS  OF  SCIENCE. 

in  the  most  unexpected  manner  the  influence  of  chemi- 
cal combination.  Thus  the  elementary  gases,  oxygen, 
hydrogen,  and  nitrogen,  and  the  mixture  atmospheric 
air,  prove  to  be  practical  vacua  to  the  rays  of  heat ; 
for  every  ray,  or,  more  strictly  speaking,  for  every 
unit  of  wave-motion,  which  any  one  of  them  inter- 
cepts, perfectly  transparent  ammonia  intercepts  5,460 
units,  defiant  gas  6,030  units,  while  sulphurous 
acid  gas  absorbs  6,480  units.  What  becomes  of  the 
wave-motion  thus  intercepted  ?  It  is  applied  to  the 
heating  of  the  absorbing  gas.  Through  air,  oxygen, 
hydrogen,  and  nitrogen,  the  waves  of  ether  pass  with- 
out absorption,  and  these  gases  are  not  sensibly  changed 
in  temperature  by  the  most  powerful  calorific  rays.  The 
position  of  nitrous  oxide  in  the  foregoing  table  is 
worthy  of  particular  notice.  In  this  gas  we  have  the 
same  atoms  in  a  state  of  chemical  union,  that  exist 
uncombined  in  the  atmosphere ;  but  the  absorption  of 
the  compound  is  1,800  times  that  of  air. 


5.  Formation  of  Invisible  FocL 

This  extraordinary  deportment  of  the  elementary 
gases  naturally  directed  attention  to  elementary  bodies 
in  other  states  of  aggregation.  Some  of  Melloni's  re- 
sults now  attained  a  new  significance.  This  celebrated 
experimenter  had  found  crystals  of  sulphur  to  be  highly 
pervious  to  radiant  heat ;  he  had  also  proved  that 
lamp-black,  and  black  glass,  (which  owes  its  blackness 
to  the  element  carbon)  were  to  a  considerable  extent 
transparent  to  calorific  rays  of  low  refrangibility.  These 
facts,  harmonising  so  strikingly  with  the  deportment  of 
the  simple  gases,  suggested  further  enquiry.  Sulphur 
dissolved  in  bisulphide  of  carbon  was  found  almost  per- 


EADIATION.  41 

fectly  diathermic.  Th«  dense  and  deeply-coloured 
element  bromine  was  examined,  and  found  competent 
to  cut  off  the  light  of  our  most  brilliant  flames,  while 
it  transmitted  the  invisible  calorific  rays  with  extieme 
freedom.  Iodine,  the  companion  element  of  bromine, 
was  next  thought  of,  but  it  was  found  impracticable  to 
examine  the  substance  in  its  usual  solid  condition.  It 
however  dissolves  freely  in  bisulphide  of  carbon.  There 
is  no  chemical  union  between  the  liquid  and  the  iodine ; 
it  is  simply  a  case  of  solution,  in  which  the  uncombined 
atoms  of  the  element  can  act  upon  the  radiant  heat. 
When  permitted  to  do  so,  it  was  found  that  a  layer  of 
dissolved  iodine,  sufficiently  opaque  to  cut  off  the  Hght 
of  the  midday  sun,  was  almost  absolutely  transparent 
to  the  invisible  calorific  rays.  ^ 

By  prismatic  analysis  Sir  William  Herschel  separated 
the  luminous  from  the  non-luminous  rays  of  the  sun, 
and  he  also  sought  to  render  the  obscure  rays  visible  by 
concentration.  Intercepting  the  luminous  portion  of 
his  spectrum  he  brought,  by  a  converging  lens,  the 
ultra-red  rays  to  a  focus,  but  by  this  condensation  he 
obtained  no  light.  The  solution  of  iodine  offers  a 
means  of  filtering  the  solar  beam,  or  failing  it,  the  beam 
of  the  electric  lamp,  which  renders  attainable  far  more 
powerful  foci  of  invisible  rays  than  could  possibly  be 
obtained  by  the  method  of  Sir  William  Herschel.  For 
to  form  his  spectrum  he  was  obliged  to  operate  upon 
solar  light  which  had  passed  through  a  narrow  slit  or 
through  a  small  aperture,  the  amount  of  the  obscure 
heat  being  limited  by  this  circumstance.  But  with 
our  opaque  solution  we  may  employ  the  entire  surface 
of  the  largest  lens,  and  having  thus  converged  the  rays, 

*  Professor  Dewar  has  recently  succeeded  in  producing  a  medium 
highly  opaque  to  light,  and  highly  transparent  to  obscure  heat,  by 
fusing  together  sulphur  and  iodine. 


42  FRAGMENTS  OF  SCIENCE. 

luminous  and  non-luminous,  we  can  intercept  the  formal 
by  the  iodine,  and  do  what  we  please  with  the  latter. 
Experiments  of  this  character,  not  only  with  the  iodine 
solution,  buii  also  with  black:  glass  and  layers  of  lamp- 
black, were  publicly  performed  at  the  Royal  Institution 
in  the  early  part  of  1862,  and  the  effects  at  the  foci  of 
invisible  rays,  then  obtained,  were  such  as  had  never 
been  witnessed  previously. 

In  the  experiments  here  referred  to,  glass  lenses 
were  employed  to  concentrate  the  rays.  But  glass, 
though  highly  transparent  to  the  luminous,  is  in  a  high 
degree  opaque  to  the  invisible,  heat-rays  of  the  electric 
lamp,  and  hence  a  large  portion  of  those  rays  was  inter- 
cepted by  the  glass.  The  obvious  remedy  here  is  to 
employ  rock-salt  lenses  instead  of  glass  ones,  or  to  aban- 
don the  use  of  lenses  wholly,  and  to  concentrate  the 
rays  by  a  metallic  mirror.  Both  of  these  improvements 
have  been  introduced,  and,  as  anticipated,  the  invisible 
foci  have  been  thereby  rendered  more  intense.  The 
mode  of  operating  remains  however  the  same,  in  prin- 
ciple, as  that  made  known  in  1862.  It  was  then  found 
that  an  instant's  exposure  of  the  face  of  the  thermo- 
electric pile  to  the  focus  of  invisible  rays,  dashed  the 
needles  of  a  coarse  galvanometer  violently  aside.  It 
is  now  found  that  on  substituting  for  the  face  of  the 
thermo-electric  pile  a  combustible  body,  the  invisible 
rays  are  competent  to  set  that  body  on  fire. 


6.  Visible  cmd  Invisible  Rays  of  the  Electric  Light. 

We  have  next  to  examine  what  proportion  the  non- 
luminous  rays  of  the  electric  light  bear  to  the  luminous 
ones.  This  the  opaque  solution  of  iodine  enables  us  to 
do  with  an  extremely  close  approximation  to  the  truth. 


RADIATION.  43 

The  pure  bisulphide  of  carbon,  which  is  the  solveni 
of  the  iodine,  is  perfectly  transparent  to  the  luminous, 
and  almost  perfectly  transparent  to  the  dark,  rays  of  the 
electric  lamp.  Supposing  the  total  radiation  of  the 
lamp  to  pass  through  the  transparent  bisulphide,  while 
through  the  solution  of  iodine  only  the  dark  rays  are 
transmitted.  If  we  determine,  by  means  of  a  thermo- 
electric pile,  the  total  radiation,  and  deduct  from  it 
the  purely  obscure,  we  obtain  the  value  of  the  purely 
luminous  emission.  Experiments,  performed  in  this 
way,  prove  that  if  all  the  visible  rays  of  the  electric 
light  were  converged  to  a  focus  of  dazzling  brilliancy, 
its  heat  would  only  be  one-eighth  of  that  produced  at 
the  unseen  focus  of  the  invisible  rays. 

Exposing  his  thermometers  to  the  successive  colours 
of  the  solar  spectrum.  Sir  William  Herschel  determined 
the  heating  power  of  each,  and  also  that  of  the  region 
beyond  the  extreme  red.  Then  drawing  a  straight  line 
to  represent  the  length  of  the  spectrum,  he  erected,  at 
various  points,  perpendiculars  to  represent  the  calorific 
intensity  existing  at  those  points.  Uniting  the  ends 
of  all  his  perpendiculars,  he  obtained  a  curve  which 
showed  at  a  glance  the  manner  in  which  the  heat  was 
distributed  in  the  solar  spectrum.  Professor  Miiller  of 
Freiburg,  with  improved  instruments,  afterwards  made 
similar  experiments,  and  constructed  a  more  accurate 
diagram  of  the  same  kind.  We  have  now  to  examine 
the  distribution  of  heat  in  the  spectrum  of  the  electric 
light ;  and  for  this  purpose  we  shall  employ  a  par- 
ticular form  of  the  thermo-electric  pile,  devised  by 
Melloni.  Its  face  is  a  rectangle,  which  by  means  of 
movable  side-pieces  can  be  rendered  as  narrow  as  de- 
sired. We  can,  for  example,  have  the  face  of  the  pile 
the  tenth,  the  hundredth,  or  even  the  thousandth  of  an 
inch  in  breadth.     By  means  of  an  endless  screw,  this 


44  FRAGMENTS  OF  SCIENCE. 

linear  thermo-electric  pile  may  be  moved  through  tho 
entire  spectrum,  from  the  violet  to  the  red,  the  amount 
of  heat  falling  upon  the  pile  at  every  point  of  its  march, 
being  declared  by  a  magnetic  needle  associated  with 
the  pile. 

When  this  instrument  is  brought  up  to  the  violet 
end  of  the  spectrum  of  the  electric  light,  the  heat  is 
found  to  be  insensible.  As  the  pile  is  gradually  moved 
from  the  violet  end  towards  the  red,  heat  soon  mani- 
fests itself,  augmenting  as  we  approach  the  red.  Of  all 
the  colours  of  the  visible  spectrum  the  red  possesses  the 
highest  heating  power.  On  pushing  the  pile  into  the 
dark  region  beyond  the  red,  the  heat,  instead  of  vanish- 
ing, rises  suddenly  and  enormously  in  intensity,  until 
at  some  distance  beyond  the  red  it  attains  a  maximum. 
Moving  the  pile  still  forward,  the  thermal  power  falls, 
somewhat  more  rapidly  than  it  rose.  It  then  gradually 
shades  away,  but,  for  a  distance  beyond  the  red  greater 
than  the  length  of  the  whole  visible  spectrum,  signs  of 
heat  may  be  detected. 

Drawing  a  datum  line,  and  erecting  along  it  per- 
pendiculars, proportional  in  length  to  the  thermal 
intensity  at  the  respective  points,  we  obtain  the  extra- 
ordinary curve,  shown  on  the  opposite  page,  which 
exhibits  the  distribution  of  heat  in  the  spectrum  of  the 
electric  light.  In  the  region  of  dark  rays,  beyond  the 
red,  the  curve  shoots  up  to  B,  in  a  steep  and  massive 
peak — a  kind  of  Matterhorn  of  heat,  which  dwarfs  the 
portion  of  the  diagram  c  d  e,  representing  the  luminous 
radiation.  Indeed  the  idea  forced  upon  the  mind  by 
this  diagram  is  that  the  light  rays  are  a  mere  insigni- 
ficant appendage  to  the  heat-rays  represented  by  the 
area  a  B  c  d,  thrown  in  as  it  were  by  nature  for  the 
purpose  of  vision. 

The  diagram  drawn  by  Professor  Miiller  to  represent 


EADIATIOK. 


45 


46  FRAGMENTS  OF  SCIENCE. 

the  distribution  of  heat  in  the  solar  spectrum  is  not  by 
any  means  so  striking  as  that  just  described,  and  the 
reason,  doubtless,  is  that  prior  to  reaching  the  earth 
the  solar  rays  have  to  traverse  our  atmosphere.  By  the 
aqueous  vapour  there  diffused,  the  summit  of  the  peak 
representing  the  sun's  invisible  radiation  is  cut  off.  A 
similar  lowering  of  the  mountain  of  invisible  heat  is 
observed  when  the  rays  from  the  electric  light  are  per- 
mitted to  pass  through  a  film  of  water,  which  acts  upon 
them  as  the  atmospheric  vapour  acts  upon  the  rays  of 
the  sun. 


7.  Combustion  by  Invisible  Rays. 

The  sun's  invisible  rays  far  transcend  the  visible 
ones  in  heating  power,  so  that  if  the  alleged  perfor- 
mances of  Archimedes  during  the  siege  of  Syracuse  had 
any  foundation  in  fact,  the  dark  solar  rays  would  have 
been  the  philosopher's  chief  agents  of  combustion.  On 
a  small  scale  we  can  readily  produce,  with  the  purely 
invisible  rays  of  the  electric  light,  all  that  Archi- 
medes is  said  to  have  performed  with  the  sun's  total 
radiation.  Placing  behind  the  electric  light  a  small 
concave  mirror,  the  rays  are  converged,  the  cone  of 
reflected  rays  and  their  point  of  convergence  being 
rendered  clearly  visible  by  the  dust  always  floating  in  the 
air.  Placing  between  the  luminous  focus  and  the  source 
of  rays  our  solution  of  iodine,  the  light  of  the  cone  is 
entirely  cut  away  ;  but  the  intolerable  heat  experienced 
when  the  hand  is  placed,  even  for  a  moment,  at  the 
dark  focus,  shows  that  the  calorific  rays  pass  unimpeded 
through  the  opaque  solution. 

Almost  anything  that  ordinary  fire  can  effect  may 
be  accomplished  at  the  focus  of  invisible  rays ;  the  ai/r 


RADIATION.  47 

at  the  focus  remaining  at  the  same  time  perfectly  cold, 
on  account  of  its  transparency  to  the  heat-rays.  An  air 
thermometer,  with  a  hollow  rock-salt  bulb,  would  be 
unaffected  by  the  heat  of  the  focus :  there  would  be  no 
expansion,  and  in  the  open  air  there  is  no  convection. 
The  ether  at  the  focus,  and  not  the  air,  is  the  sub- 
stance in  which  the  heat  is  embodied.  A  block  of  wood, 
placed  at  the  focus,  absorbs  the  heat,  and  dense  volumes 
of  smoke  rise  swiftly  upwards,  showing  the  manner  in 
which  the  air  itself  would  rise,  if  the  invisible  rays  were 
competent  to  heat  it.  At  the  perfectly  dark  focus  dry 
paper  is  instantly  inflamed :  chips  of  wood  are  speedily 
burnt  up  :  lead,  tin,  and  zinc  are  fused :  and  disks  of 
charred  paper  are  raised  to  vivid  incandescence.  It 
might  be  supposed  that  the  obscure  rays  would  show 
no  preference  for  black  over  white ;  but  they  do  show 
a  preference,  and  to  obtain  rapid  combustion,  the  body, 
if  not  already  black,  ought  to  be  blackened.  When 
metals  are  to  be  burned,  it  is  necessary  to  blacken  or 
otherwise  tarnish  them,  so  as  to  diminish  their  reflective 
power.  Blackened  zinc  foil,  when  brought  into  the 
focus  of  invisible  rays,  is  instantly  caused  to  blaze,  and 
burns  with  its  peculiar  purple  light.  Magnesium  wire 
flattened,  or  tarnished  magnesium  ribbon,  also  bursts 
into  flame.  Pieces  of  charcoal  suspended  in  a  receiver 
full  of  oxygen  are  also  set  on  fire  when  the  invisible 
focus  falls  upon  them ;  the  dark  rays  after  having 
passed  through  the  receiver,  still  possessing  suflScient 
power  to  ignite  the  charcoal,  and  thus  initiate  the 
attack  of  the  oxygen.  If,  instead  of  being  plunged  in 
oxygen,  the  charcoal  be  suspended  in  vacuo,  it  immedi- 
ately glows  at  the  place  where  the  focus  fe.lls. 


48  FRAGMENTS  OF  SCIENCE. 


8.  Transmutation  of  Bays : '  Galoreseence, 

Eminent  experimenters  were  long  occupied  in  de- 
monstrating the  substantial  identity  of  light  and 
radiant  heat,  and  we  have  now  the  means  of  offering 
a  new  and  striking  proof  of  this  identity.  A  concave 
mirror  produces,  beyond  the  object  which  it  reflects, 
an  inverted  and  magnified  image  of  the  object. 
Withdrawing,  for  example,  our  iodine  solution,  an  in- 
tensely luminous  inverted  image  of  the  carbon  points  of 
the  electric  light  is  formed  at  the  focus  of  the  mirror 
employed  in  the  foregoing  experiments.  When  the 
solution  is  interposed,  and  the  light  is  cut  away,  what 
becomes  of  this  image  ?  It  disappears  from  sight ; 
but  an  invisible  thermograph  remains,  and  it  is  only 
the  peculiar  constitution  of  our  eyes  that  disqualifies 
us  from  seeing  the  picture  formed  by  the  calorific  rays. 
Falling  on  white  paper,  the  image  chars  itself  out: 
falling  on  black  paper,  two  holes  are  pierced  in  it, 
corresponding  to  the  images  of  the  two  coke  points  : 
but  falling  on  a  thin  plate  of  carbon  in  vacuo,  or  upon 
a  thin  sheet  of  platinised  platinum,  either  in  vacuo  or  in 
air,  radiant  heat  is  converted  into  light,  and  the  image 
stamps  itself  in  vivid  incandescence  upon  both  the  car- 
bon and  the  metal.  Results  similar  to  those  obtained 
with  the  electric  light  have  also  been  obtained  with 
the  invisible  rays  of  the  lime-light  and  of  the  sun. 

Before  a  Cambridge  audience  it  is  hardly  necessary 
to  refer  to  the  excellent  researches  of  Professor  Stokes 
at  the  opposite  end  of  the  spectrum.  The  above  re- 
sults constitute  a  kind  of  complement  to  his  discoveries. 
Professor  Stokes  named  the  phenomena  which  he  has 

»  I  borrow  this  term  from  Professor  Challis,  'Philosophical 
Magazine»*  voL  xii.  p.  62L 


RADIATION.  49 

discovered  and  investigated  Fluorescence ;  for  the  new 
phenomena  here  described  I  have  proposed  the  term 
Galorescence,  He,  by  the  interposition  of  a  proper 
medium,  so  lowered  the  refrangibility  of  the  ultra- 
violet rays  of  the  spectrum  as  to  render  them  visible. 
Here,  by  the  interposition  of  the  platinum  foil,  the 
refrangibility  of  the  ultra-red  rays  is  so  exalted  as  to 
render  them  visible.  Looking  through  a  prism  at  the 
incandescent  image  of  the  carbon  points,  the  light  of 
the  image  is  decomposed,  and  a  complete  spectrum  is 
obtained.  The  invisible  rays  of  the  electric  light, 
remoulded  by  the  atoms  of  the  platinum,  shine  thus 
visibly  forth ;  ultra-red  rays  being  converted  into  red, 
orange,  yellow,  green,  blue,  indigo,  violet,  and  ultra- 
violet ones.  Could  we,  moreover,  raise  the  original 
source  of  rays  to  a  sufficiently  high  temperature,  we 
might  not  only  obtain  from  the  dark  rays  of  such  a 
source  a  single  incandescent  image,  but  from  the  dark 
rays  of  this  image  we  might  obtain  a  second  one,  from  the 
dark  rays  of  the  second  a  third,  and  so  on — a  series  of 
complete  images  and  spectra  being  thus  extracted  from 
the  invisible  emission  of  the  primitive  source.* 

>  On  investigating  the  calorescence  produced  by  rays  transmitted 
through  glasses  of  various  colours,  it  was  found  that  in  the  case  of 
certain  specimens  of  blue  glass,  the  platinum  foil  glowed  with  a 
pi?ik  or  jfitrplish  light.  The  effect  was  not  subjective,  and  consider- 
ations of  obvious  interest  are  suggested  by  it.  Different  kinds  of 
black  glass  differ  notably  as  to  their  power  of  transmitting  radiant 
heat.  When  thin,  some  descriptions  tint  the  sun  with  a  greenish 
hue :  others  make  it  appear  a  glowing  red  without  any  trace  of 
green.  The  latter  are  far  more  diathermic  than  the  former.  In 
fact,  carbon  when  perfectly  dissolved  and  incorporated  with  a  good 
white  glass,  is  highly  transparent  to  the  calorific  rays,  and  by  eni- 
ploying  it  as  an  absorbent  the  phenomena  of  *  calorescence '  may  be 
obtained,  though  in  a  less  striking  form  than  with  the  iodine.  The 
black  glass  chosen  for  thermometers,  and  intended  to  absorb  com- 
pletely the  solar  heat,  may  entirely  fail  in  this  object,  if  the  glass 
in  which  the  carbon  is  incorporated  be  colourless.    To  render  the 


50  FBAGMENTS  OF  SCIENCE. 

9.  Deadness  of  the  Optic  Nerve  to  the  Calorific  Raya. 

The  layer  of  iodine  used  in  the  foregoing  experi- 
ments intercepted  the  rays  of  the  noonday  sun.  No 
trace  of  light  from  the  electric  lamp  was  visible  in  the 
darkest  room,  even  when  a  white  screen  was  placed  at 
the  focus  of  the  mirror  employed  to  concentrate  the 
light.  It  was  thought,  however,  that  if  the  retina 
itself  were  brought  into  the  focus  the  sensation  of  light 
might  be  experienced.  The  danger  of  this  experiment 
was  twofold.  If  the  dark  rays  were  absorbed  in  a  high 
degree  by  the  humours  of  the  eye  the  albumen  of  the 
humours  might  coagulate  along  the  line  of  the  rays.  If, 
on  the  contrary,  no  such  high  absorption  took  place,  the 
rays  might  reach  the  retina  with  a  force  suJBficient  to  de- 
stroy it.  To  test  the  likelihood  of  these  results,  experi- 
ments were  made  on  water  and  on  a  solution  of  alum,  and 
they  showed  it  to  be  very  improbable  that  in  the  brief 
time  requisite  for  an  experiment  any  serious  damage 
could  be  done.  The  eye  was  therefore  caused  to 
approach  the  dark  focus,  no  defence,  in  the  first  in- 
stance, being  provided ;  but  the  heat,  acting  upon  the 
parts  surrounding  the  pupil,  could  not  be  borne.  An 
aperture  was  therefore  pierced  in  a  plate  of  metal,  and 
the  eye,  placed  behind  the  aperture,  was  caused  to 
approach  the  point  of  convergence  of  invisible  rays. 
The  focus  was  attained,  first  by  the  pupil  and  after- 
wards by  the  retina.  Eemoving  the  eye,  but  per- 
mitting the  plate  of  metal  to  remain,  a  sheet  of 
platinum  foil  was  placed  in  the  position  occupied  by  the 

bulb  of  a  thermometer  a  perfect  absorbent,  the  glass  ought  in  the 
first  instance  to  be  green.  Soon  after  the  discovery  of  fluorescence 
the  late  Dr.  "William  Allen  Miller  pointed  to  the  lime-light  as  an 
illustration  of  exalted  refrangibility.  Direct  experiments  have 
since  entirely  confirmed  the  view  expressed  at  page  210  of  his  work 
on  *  Chemistry,'  published  in  1855, 


RADIATION.  61 

retina  a  moment  before.  The  platinum  became  red-hot. 
No  sensible  damage  was  done  to  the  eye  by  this  experi- 
ment ;  no  impression  of  light  was  produced ;  the  optic 
nerve  was  not  even  conscious  of  heat. 

But  the  humours  of  the  eye  are  known  to  be  highly 
impervious  to  the  invisible  calorific  rays,  and  the 
question  therefore  arises,  'Did  the  radiation  in  the 
foi'egoing  experiment  reach  the  retina  at  all  ? '  The 
answer  is,  that  the  rays  were  in  part  transmitted  to  the 
retina,  and  in  part  absorbed  by  the  humours.  Experi- 
ments on  the  eye  of  an  ox  showed  that  the  proportion 
of  obscure  rays  which  reached  the  retina  amounted  to 
18  per  cent,  of  the  total  radiation;  while  the  luminous 
emission  from  the  electric  light  amounts  to  no  more 
than  1 0  per  cent,  of  the  same  total.  Were  the  purely 
luminous  rays  of  the  electric  lamp  converged  by  our 
mirror  to  a  focus,  there  can  be  no  doubt  as  to  the  fate 
of  a  retina  placed  there.  Its  ruin  would  be  inevitable ; 
and  yet  this  would  be  accomplished  by  an  amount  of 
wave-motion  but  little  more  than  half  of  that  which 
the  retina,  without  exciting  consciousness,  bears  at  the 
focus  of  invisible  rays. 

This  subject  will  repay  a  moment's  further  attention. 
At  a  common  distance  of  a  foot  the  visible  radiation  of 
the  electric  light  employed  in  these  experiments  is 
800  times  the  light  of  a  candle.  At  the  same  distance, 
the  portion  of  the  radiation  of  the  electric  light  which 
reaches  the  retina,  but  fails  to  excite  vision,  is  about 
1,500  times  the  luminous  radiation  of  the  candle.* 
But  a  candle  on  a  clear  night  can  readily  be  seen  at  a 
distance  of  a  mile,  its  light  at  this  distance  being  less 
^^^^  ao.000,000  ^^  ^^s   light  at  the   distance  of  a  foot. 

*  It  will  be  borne  in  mind  that  the  heat  which  any  ray,  luminous 
or  non- luminous,  is  competent  to  genei^te  ig  the  true  measure  of 
the  energy  of  the  ray. 


52  FEAGMENTS  OF  SCIENCE. 

Hence,  to  make  the  candle-light  a  mile  oflp  equal  in 
power  to  the  non-luminous  radiation  received  from  the 
electric .  light  at  a  foot  distance,  its  intensity  would 
have  to  be  multiplied  by  1,500x20,000,000,  or  by 
thirty  thousand  millions.  Thus  the  thirty  thousand 
millionth  part  of  the  invisible  radiation  from  the 
electric  light,  received  by  the  retina  at  the  distance  of 
a  foot,  would,  if  slightly  changed  in  character,  be  amply 
sufficient  to  provoke  vision.  Nothing  could  more 
forcibly  illustrate  that  special  relationship  supposed  by 
Melloni  and  others  to  subsist  between  the  optic  nerve 
and  the  oscillating  periods  of  luminous  bodies.  The 
optic  nerve  responds,  as  it  were,  to  the  waves  with 
which  it  is  in  consonance,  while  it  refuses  to  be  excited 
by  others  of  almost  infinitely  greater  energy,  whose 
periods  of  recurrence  are  not  in  unison  with  its  own. 


10.  Persistence  of  Rays, 

At  an  early  part  of  this  lecture  it  was  affirmed,  that 
when  a  platinum  wire  was  gradually  raised  to  a  state 
of  high  incandescence,  new  rays  were  constantly  added, 
while  the  intensity  of  the  old  ones  was  increased.  Thus, 
in  Br.  Draper's  experiments,  the  rise  of  temperature 
that  generated  the  orange,  yellow,  green,  and  blue 
augmented  the  intensity  of  the  red.  What  is  true  of 
the  red  is  true  of  every  other  ray  of  the  spectrum, 
visible  and  invisible.  We  cannot  indeed  see  the  aug- 
mentation of  intensity  in  the  region  beyond  the  red, 
but  we  can  measure  it  and  express  it  numerically. 
With  this  view  the  following  experiment  was  performed : 
A  spiral  of  platinum  wire  was  surrounded  by  a  small 
glass  globe  to  protect  it  from  currents  of  air ;  through 
an  orifice  in  the  globe  the  rays  could  pass  from  the 


KADIATION. 


53 


spiral  and  fall  afterwards  upon  a  thermo-electric  pile. 
Placing  in  front  of  the  orifice  an  opaque  solution  of 
iodine,  the  platinum  svas  gradually  raised  from  a  low 
dark  heat  to  the  fullest  incandescence,  with  the  follow- 
ing results : — 


Appearance 

Energy  of 

of  spiral                                                obscure  radiation 

Dark 1 

Dark,  but  hotter    , 

3 

Dark,  but  still  hotter 

6 

Dark,  but  still  hotter    . 

10 

Feeble  red     , 

19 

Dull  red 

.        25 

Red        ...        , 

87 

Full  red.        .        .        . 

62 

Orange  . 

89 

Bright  orange 

144 

Yellow  . 

.      202 

White    . 

,      276 

Intense  white        •        , 

440 

Thus  the  augmentation  of  the  electric  current, 
which  raises  the  wire  from  its  primitive  dark  condition 
to  an  intense  white  heat,  exalts  at  the  same  time  the 
energy  of  the  obscure  radiation,  until  at  the  end  it  is 
fully  440  times  what  it  was  at  the  beginning. 

What  has  been  here  proved  true  of  the  totality  of 
the  ultra-red  rays  is  true  for  each  of  them  singly. 
Placing  our  linear  thermo-electric  pile  in  any  part  of 
the  ultra-red  spectrum,  it  may  be  proved  that  a  ray 
once  emitted  continues  to  be  emitted  with  increased 
energy  as  the  temperature  is  augmented.  The  platinum 
spiral,  so  often  referred  to,  being  raised  to  whiteness 
by  an  electric  current,  a  brilliant  spectrum  was  formed 
from  its  light.  A  linear  thermo-electric  pile  was  placed 
in  the  region  of  obscure  rays  beyond  the  red,  and  by 
diminishing  the  current  the  spiral  was  reduced  to  a  low 
temperature.      It  was   then   caused   to   pass   through 


54  FRAGMENTS  OF  SCIENCE. 

various  degrees  of  darkness  and  incandescence,  with 
the  following  results : — • 

Appearance  Energy  of 

of  spiral  obscure  rays 

Dark 1 

Dark 6 

Faint  red 10 

Dull  red 13 

Red 18 

Full  red 27 

Orange  .        .        ,        .        ,        ,        60 

Yellow 93 

White 122 

Here,  as  in  the  former  case,  the  dark  and  bright 
radiations  reached  their  maximum  together;  as  the 
one  augmented,  the  other  augmented,  until  at  last  the 
energy  of  the  obscure  rays  of  the  particular  refrangi- 
bility  here  chosen,  became  122  times  what  it  was  at 
first.  ,To  reach  a  white  heat  the  wire  has  to  pass 
through  all  the  stages  of  invisible  radiation,  but  in  its 
most  brilliant  condition  it  embraces,  in  an  intensified 
form,  the  rays  of  all  those  stages. 

And  thus  it  is  with  all  other  kinds  of  matter,  as  far 
as  they  have  hitherto  been  examined.  Coke,  whether 
brought  to  a  white  heat  by  the  electric  current,  or  by 
the  oxyhydrogen  jet,  pours  out  invisible  rays  with 
augmented  energy,  as  its  light  is  increased.  The  same 
is  true  of  lime,  bricks,  and  other  substances.  It  is 
true  of  all  metals  which  are  capable  of  being  heated  to 
incandescence.  It  also  holds  good  for  phosphorus 
burning  in  oxygen.  Every  gush  of  dazzb'ng  light  has 
associated  with  it  a  gush  of  invisible  radiant  heat, 
which  far  transcends  the  light  in  energy.  This  con- 
dition of  things  applies  to  all  bodies  capable  of  being 
raised  to  a  white  heat,  either  in  the  solid  or  the  molten 
condition.      It    would    doubtless    also    apply   to   the 


RADIATION.  55 

luminous  fogs  formed  by  the  condensation  of  incan- 
descent vapours.  In  such  cases  when  the  curve  repre- 
senting the  radiant  energy  of  the  body  is  constructed, 
the  obscure  radiation  towers  upwards  like  a  mountain, 
the  huninous  radiation  resembling  a  mere  '  spur  '  at  its 
base.  From  the  very  brightness  of  the  light  of  some  of 
the  fixed  stars  we  may  infer  the  intensity  of  that  dark 
radiation,  which  is  the  precursor  and  inseparable  asso- 
ciate of  their  luminous  rays. 

We  thus  find  the  luminous  radiation  appearing 
when  the  radiant  body  has  attained  a  certain  tem- 
perature ;  or,  in  other  words,  when  the  vibrating  atoms 
of  the  body  have  attained  a  certain  width  of  swing. 
In  solid  and  molten  bodies  a  certain  amplitude  cannot 
be  surpassed  without  the  introduction  of  periods  of 
vibration,  which  provoke  the  sense  of  vision.  How  are 
we  to  figure  this  ?  If  permitted  to  speculate,  we  might 
ask,  are  not  these  more  rapid  vibrations  the  progeny  of 
the  slower  ?  Is  it  not  really  the  mutual  action  of  the 
atoms,  when  they  swing  through  very  wide  spaces,  and 
thus  encroach  upon  each  other,  that  causes  them  to 
tremble  in  quicker  periods  ?  If  so,  whatever  be  the 
agency  by  which  the  large  swinging  space  is  obtained, 
we  shall  have  light-giving  vibrations  associated  with  it. 
It  matters  not  whether  the  large  amplitudes  be  pro- 
duced by  the  strokes  of  a  hammer,  or  by  the  blows  of 
the  molecules  of  a  non-luminous  gas,  like  air  at  some 
height  above  a  gas-flame  ;  or  by  the  shock  of  the  ether 
particles  when  transmitting  radiant  heat.  The  result 
in  all  cases  will  be  incandescence.  Thus,  the  invisible 
waves  of  our  filtered  electric  beam  may  be  regarded  as 
generating  synchronous  vibrations  among  the  atoms  of 
the  platinum  on  which  they  impinge  ;  but,  once  these 
vibrations  have  attained  a  certain  amplitude,  the 
mutual  jostling  of  the  atoms  produces  quicker  tremors, 
5 


56  FEAGMENTS  OF  SCIENCE. 

and  the  ligbt-giving  waves  follow  as  the  necessary 
product  of  the  heat-giving  ones. 


11.  Absorption  of  Radiant  Heat  by  Vapours  amd 

Odours, 

We  commenced  the  demonstrations  brought  forward 
in  this  lecture  by  experiments  on  permanent  gases,  and 
we  have  now  to  turn  our  attention  to  the  vapours  of 
volatile  liquids.  Here,  as  in  the  case  of  the  gases, 
vast  differences  have  been  proved  to  exist  between 
various  kinds  of  molecules,  as  regards  their  power  of 
intercepting  the  calorific  waves.  While  some  vapours 
allow  the  waves  a  comparatively  free  passage,  the 
faintest  mixture  of  other  vapours  causes  a  deflection  of 
the  magnetic  needle.  Assuming  the  absorption  effected 
by  air,  at  a  pressure  of  one  atmosphere,  to  be  unity, 
the  following  are  the  absorptions  effected  by  a  series  of 
vapours  at  a  pressure  of  -^t^  of  an  atmosphere : — 


Name  of  vapour 

Absorpt 

Bisulphide  of  carbon     . 

47 

Iodide  of  methyl   . 

.       115 

Benzol 

136 

Amylene        .        .        .        • 

.      321 

Sulphuric  ether      .        •        • 

.      440 

Formic  ether. 

.      548 

Acetic  ether  .... 

.      612 

Bisulphide  of  carbon  is  the  most  transparent  vapour 
in  this  list ;  and  acetic  ether  the  most  opaque ;  -g^th  of 
an  atmosphere  of  the  former,  however,  produces  47 
times  the  effect  of  a  whole  atmosphere  of  air,  while  -^th 
of  an  atmosphere  of  the  latter  produces  612  times  the 
effect  of  a  whole  atmosphere  of  air.  Keducing  dry  air 
to  the  pressure  of  the  acetic  ether  here  employed,  and 
comparing  them  then  together,  the  quantity  of  wave- 


RADIATION.  57 

motion  intercepted  by  the  ether  would  be  many  thousand 
times  that  intercepted  by  the  air. 

Any  one  of  these  vapours  discharged  into  the  free 
atmosphere,  in  front  of  a  body  emitting  obscure  rays, 
intercepts  more  or  less  of  the  radiation.  A  similar 
effect  is  produced  by  perfumes  diffused  in  the  air, 
though  their  attenuation  is  known  to  be  almost  infinite. 
Carrying,  for  example,  a  current  of  dry  air  over  bibu- 
lous paper,  moistened  by  patchouli,  the  scent  taken  up 
by  the  current  absorbs  30  times  the  quantity  of  heat 
intercepted  by  the  air  which  carries  it ;  and  yet  pat- 
chouli acts  more  feebly  on  radiant  heat  than  any  other 
perfume  yet  examined.  Here  follow  the  results  ob- 
tained with  various  essential  oils,  the  odour,  in  each 
case,  being  carried  by  a  current  of  dry  air  into  the 
tube  already  employed  for  gases  and  vapours  : — 

Name  of  perfume  Absorption 

Patchouli 30 

Sandalwood.        ,        .  -     .        .  32 

Geranium .33 

Oil  of  cloves 34 

Otto  of  roses .....        37 

^  Bergamot 44 

Neroli 47  t 

Lavender 60 

Lemon 65 

Portugal 67 

Thyme 68 

Rosemary 74 

Oil  of  laurel 80 

Camomile  flowers ....        87 

Cassia    .        .  ...  109 

Spikenard      .        •        .        •        .  365 

Aniseed.        •       •        .        •        .  372 

Thus  the  absorption  by  a  tube  full  of  dry  air  being 
1,  that  of  the  odour  of  patchouli  diffused  in  it  is  30, 
that  of  lavender  60,  that  of  rosemary  74,  whilst  that  of 
aniseed  amounts  to  372.  It  would  be  idle  to  speculate 
on  the  quantities  of  matter  concerned  in  these  actions. 


58  FEAGMENTS  OF  SCIENCE. 


12.  Aqueous  Vapour  in  relation  to  the  Terrestrial 

Temperatures, 

We  are  now  fully  prepared  for  a  result  which, 
without  such  preparation,  might  appear  incredible. 
Water  is,  to  some  extent,  a  volatile  body,  and  our 
atmosphere,  resting  as  it  does  upon  the  surface  of  the 
ocean,  receives  from  it  a  continual  supply  of  aqueous 
vapour.  It  would  be  an  error  to  confound  clouds  or 
fog  or  any  visible  mist  with  the  vapour  of  water,  which 
is  a  perfectly  impalpable  gas,  diffused,  even  on  the 
clearest  days,  throughout  the  atmosphere.  Compared 
with  the  great  body  of  the  air,  the  aqueous  vapour  it 
contains  is  of  almost  infinitesimal  amount,  99^  out  of 
every  100  parts  of  the  atmosphere  being  composed  of 
oxygen  and  nitrogen.  In  the  absence  of  experiment, 
we  should  never  think  of  ascribing  to  this  scant  and 
varying  constituent  any  important  influence  on  terres- 
trial radiation ;  and  yet  its  influence  is  far  more  potent 
than  that  of  the  great  body  of  the  air.  To  say  that  on 
a  day  of  average  humidity  in  England,  the  atmospheric 
vapour  exerts  100  times  the  action  of  the  air  itself, 
would  certainly  be  an  understatement  of  the  fact. 
Comparing  a  single  molecule  of  aqueous  vapour  with 
an  atom  of  either  of  the  main  constituents  of  our 
atmosphere,  I  am  not  prepared  to  say  how  many 
thousand  times  the  action  of  the  former  exceeds  that  of 
the  latter. 

But  it  must  be  borne  in  mind  that  these  large 
numbers  depend,  in  part,  on  the  extreme  feebleness  of 
the  air;  the  power  of  aqueous  vapour  seems  vast, 
because  that  of  the  air  with  which  it  is  compared  is 
infinitesimal.  Absolutely  considered,  however,  this 
substance,  notwithstanding  its  small  specific  gravity, 


RADIATION.  59 

exercises  a  very  potent  action.  Probably  from  10  to 
15  per  cent,  of  the  heat  radiated  from  the  earth  is 
absorbed  within  10  or  20  feet  of  the  earth's  surface. 
This  must  evidently  be  of  the  utmost  consequence  to 
the  life  of  the  world.  Imagine  the  superficial  molecules 
of  the  earth  agitated  with  the  motion  of  heat,  and 
imparting  it  to  the  surrounding  ether;  this  motion 
would  be  carried  rapidly  away,  and  lost  for  ever  to  our 
planet,  if  the  waves  of  ether  had  nothing  but  the  air 
to  contend  with  in  their  outward  course.  But  the 
aqueous  vapour  takes  up  the  motion,  and  becomes 
thereby  heated,  thus  wrapping  the  earth  like  a  warm 
garment,  and  protecting  its  surface  from  the  deadly 
chill  which  it  would  otherwise  sustain.  Various  philo- 
sophers have  speculated  on  the  influence  of  an  atmo- 
spheric envelope.  De  Saussure,  Fourier,  M.  Pouillet, 
and  Mr.  Hopkins  have,  one  and  all,  enriched  scientific 
literature  with  contributions  on  this  subject,  but  the 
considerations  which  these  eminent  men  have  applied 
to  atmospheric  air,  have,  if  my  experiments  be  correct, 
to  be  transferred  to  the  aqueous  vapour. 

The  observations  of  meteorologists  furnish  impor- 
tant, though  hitherto  unconscious  evidence  of  the 
influence  of  this  agent.  Wherever  the  air  is  dry  we 
are  liable  to  daily  extremes  of  temperature.  By  day, 
in  such  places,  the  sim's  heat  reaches  the  earth  unim- 
peded, and  renders  the  maximum  high  ;  by  night,  on 
the  other  hand,  the  earth's  heat  escapes  unhindered 
into  space,  and  renders  the  minimum  low.  Hence  the 
difference  between  the  maximum  and  minimum  is 
greatest  where  the  air  is  driest.  In  the  plains  of  India, 
on  the  heights  of  the  Himalaya,  in  central  Asia,  in 
Australia — wherever  drought  reigns,  we  have  the  heat 
of  day  forcibly  contrasted  with  the  chill  of  night.  In 
the  Sahara  itself,  when  the  sun's  rays  cease  to  impinge 


60  FRAGMENTS  OF  SCIENCE. 

on  the  burning  soil,  the  temperature  runs  rapidly  down 
to  freezing,  because  there  is  no  vapour  overhead  to 
check  the  calorific  drain.  And  here  another  instance 
might  be  added  to  the  numbers  already  known,  in 
which  nature  tends  as  it  were  to  check  her  own  excess. 
By  nocturnal  refrigeration,  the  aqueous  vapour  of  the 
air  is  condensed  to  water  on  the  surface  of  'the  earth  ; 
and,  as  only  the  superficial  portions  radiate,  the  act  of 
condensation  makes  water  the  radiating  body.  Now 
experiment  proves  that  to  the  rays  emitted  by  water, 
aqueous  vapour  is  especially  opaque.  Hence  the  very 
act  of  condensation,  consequent  on  terrestrial  cooling, 
becomes  a  safeguard  to  the  earth,  imparting  to  its 
radiation  that  particular  character  which  renders  it 
most  liable  to  be  prevented  from  escaping  into  space. 

It  might  however  be  urged  that,  inasmuch  as  we 
derive  all  our  heat  from  the  sun,  the  selfsame  covering 
which  protects  the  earth  from  chill  must  also  shut  out 
the  solar  radiation.  This  is  partiaUy  true,  but  only 
partially ;  the  sun's  rays  are  different  in  quality  from 
the  earth's  rays,  and  it  does  not  at  all  follow  that  the 
substance  which  absorbs  the  one  must  necessarily  absorb 
the  other.  Through  a  layer  of  water,  for  example,  one 
tenth  of  an  inch  in  thickness,  the  sun's  rays  are  trans- 
mitted with  comparative  freedom  ;  but  through  a  layer 
half  this  .thickness,  as  Melloni  has  proved,  no  single  ray 
from  the  warmed  earth  could  pass.  In  like  manner, 
the  sun's  rays  pass  with  comparative  freedom  through 
the  aqueous  vapour  of  the  air :  the  absorbing  power  of 
this  substance  being  mainly  exerted  upon  the  invisible 
heat  that  endeavours  to  escape  from  the  earth.  In 
consequence  of  this  differential  action  upon  solar  and 
terrestrial  heat,  the  mean  temperature  of  our  planet  is 
higher  than  is  due  to  its  distance  from  the  sun. 


RADIATION.  61 


13.  Liquids  and  thei/r  Vapours  i/n  relation  to 
Radiant  Heat. 

The  deportment  here  assigned  to  atmospheric 
vapour  has  been  established  by  direct  experiments  on 
air  taken  from  the  streets  and  parks  of  London,  from 
the  downs  of  Epsom,  from  the  hills  and  sea-beach  of 
the  Isle  of  Wight,  and  also  by  experiments  on  air  in 
the  first  instance  dried,  and  afterwards  rendered  arti- 
ficially humid  by  pure  distilled  water.  It  has  also 
been  established  in  the  following  way:  Ten  volatile 
liquids  were  taken  at  random  and  the  power  of  these 
liquids,  at  a  common  thickness,  to  intercept  the  waves 
of  heat,  wa»  carefully  determined.  The  vapours  of  the 
liquids  were  next  taken,  in  quantities  proportional  to 
the  quantities  of  liquid,  and  the  power  of  the  vapours 
to  intercept  the  waves  of  heat  was  also  determined. 
Commencing  with  the  substance  which  exerted  the 
least  absorptive  power,  and  proceeding  onwards  to  the 
most  energetic,  the  following  order  of  absorption  was 
observed : — 

LIqnidfl  Vapours 

Bisulphide  of  carbon.  Bisulphide  of  carbon. 

Chloroform.  Chloroform. 

Iodide  of  methyL  Iodide  of  methyl. 

Iodide  of  ethyL  Iodide  of  ethyl. 

Benzol.  Benzol. 

Amylene.  Amylene. 

Sulphuric  ether.  Sulphuric  ether. 

Acetic  ether.  Acetic  ether. 

Formic  ether.  Formic  ether. 

AlcohoL  Alcohol. 
Water. 

We  here  find  the  order  of  absorption  in  both  cases 
to  be  the  same.  We  have  liberated  the  molecules  from 
the  bonds  which  trammel  them  more  or  less  in  a  liquid 
condition ;  but  this  change  in  their  state  of  aggregation 


62  FRAGMENTS  OF  SCIENCE. 

does  not  change  their  relative  powers  of  absorption. 
Nothing  could  more  clearly  prove  that  the  act  of  ab- 
sorption depends  upon  the  individual  molecule,  which 
equally  asserts  its  power  in  the  liquid  and  the  gaseous 
state.  We  may  safely  conclude  from  the  above  table 
that  the  position  of  a  vapour  is  determined  by  that  of 
its  liquid.  Now  at  the  very  foot  of  the  list  of  liquids 
stands  water,  signalising  itself  above  all  others  by  its 
enormous  power  of  absorption.  And  from  this  fact, 
even  if  no  direct  experiment  on  the  vapour  of  water 
had  ever  been  made,  we  should  be  entitled  to  rank  that 
vapour  as  our  most  powerful  absorber  of  radiant  heat. 
Its  attenuation,  however,  diminishes  its  action.  I  have 
proved  that  a  shell  of  air  two  inches  in  tliickness  sur- 
rounding our  planet,  and  saturated  with  the  vapour  of 
sulphuric  ether,  would  intercept  35  per  cent,  of  the 
earth's  radiation.  And  though  the  quantity  of  aqueous 
vapour  necessary  to  saturate  air  is  much  less  than  the 
amount  of  sulphuric  ether  vapour  which  it  can  sustain, 
it  IS  still  extremely  probable  that  the  estimate  already 
made  of  the  action  of  atmospheric  vapour  within  10 
feet  of  the  earth's  surface,  is  under  the  mark  ;  and  that 
we  are  indebted  to  this  wonderful  substance,  to  an 
extent  not  accurately  determined,  but  certainly  far 
beyond  what  has  hitherto  been  imagined,  for  the  tem- 
perature now  existing  at  the  surface  of  the  globe. 


14.  Reciprocity  of  Radiation  arid  Absorption, 

Throughout  the  reflections  which  have  hitherto  oc- 
cupied us,  the  image  before  the  mind  has  been  that  of 
a  radiant  source  sending  forth  calorific  waves,  which  on 
passing  among  the  molecules  of  a  gas  or  vapour  were 
intercepted  by  those  molecules  in  various  degrees.     In 


RADIATION.  63 

all  cases  it  was  the  transference  of  motion  from  the 
ether  to  the  comparatively  quiescent  molecules  of  the 
gas  or  vapour  that  occupied  our  thoughts.  We  have  now 
to  change  the  form  of  our  conception,  and  to  i&gure  these 
molecules  not  as  absorbers  but  as  radiators,  not  as  the  re- 
cipients but  as  the  originators  of  wave-motion.  That  is 
to  say,  we  must  figure  them  vibrating,  and  generating  in 
the  surrounding  ether  undulations  which  speed  through 
it  with  the  velocity  of  light.  Our  object  now  is  to 
enquire  whether  the  act  of  chemical  combination,  which 
proves  so  potent  as  regards  the  phenomena  of  absorption, 
does  not  also  manifest  its  power  in  the  phenomena  of 
radiation.  For  the  examination  of  this  question  it  is 
necessary,  in  the  first  place,  to  heat  our  gases  and 
vapours  to  the  same  temperature,  and  then  examine 
their  power  of  discharging  the  motion  thus  imparted  to 
them  upon  the  ether  in  which  they  swing. 

A  heated  copper  ball  was  placed  above  a  ring  gas- 
burner  possessing  a  great  number  of  small  apertures, 
the  burner  being  connected  by  a  tube  with  vessels  con- 
taining the  various  gases  to  be  examined.  By  gentle 
pressure  the  gases  were  forced  through  the  orifices  of 
the  burner  against  the  copper  ball,  where  each  of  them, 
being  heated,  rose  in  an  ascending  column.  A  thermo- 
electric pile,  entirely  screened  from  the  hot  ball,  was 
exposed  to  the  radiation  of  the  warm  gas,  while  the 
deflection  of  a  magnetic  needle  connected  with  the  pile 
declared  the  energy  of  the  radiation. 

By  this  mode  of  experiment  it  was  proved  that  the 
selfsame  molecular  arrangement  which  renders  a  gas  a 
powerful  absorber,  renders  it  a  powerful  radiator — that 
the  atom  or  molecule  which  is  competent  to  intercept 
the  calorific  waves  is,  in  the  same  degree,  competent  to 
send  them  forth.  Thus,  while  the  atoms  of  elementary 
gases  proved  themselves  unable  to  emit  any  sensible 


64  FRAGMENTS  OF  SCIENCE. 

amount  of  radiant  heat,  the  molecules  of  compound 
gases  were  shown  to  be  capable  of  powerfully  disturbing 
the  surrounding  ether.  By  special  modes  of  experiment 
the  same  was  proved  to  hold  good  for  the  vapours  of 
volatile  liquids,  the  radiative  power  of  every  vapour 
being  found  proportional  to  its  absorptive  power. 

The  method  of  experiment  here  pursued,  though 
not  of  the  simplest  character,  is  still  easy  to  grasp. 
When  air  is  permitted  to  rush  into  an  exhausted  tube, 
the  temperature  of  the  air  is  raised  to  a  degree  equi- 
valent to  the  vis  viva  extinguished.'  Such  air  is  said 
to  be  dynamically  heated,  and,  if  pure,  it  shows  itself 
incompetent  to  radiate,  even  when  a  rock-salt  window 
is  provided  for  the  passage  of  its  rays.  But  if  instead 
of  being  empty  the  tube  contain  a  small  quantity  of 
vapour,  the  warmed  air  communicates  its  heat  by  con- 
tact to  the  vapour,  the  molecules  of  which  convert  into 
the  radiant  form  the  heat  imparted  to  them  by  the 
atoms  of  the  air.  By  this  process  also,  which  I  have 
called  Dynamic  Kadiation,  the  reciprocity  of  radiation 
and  absorption  has  been  conclusively  proved.^ 

In  the  excellent  researches  of  Leslie,  De  la  Pro- 
vostaye  and  Desains,  and  Balfour  Stewart,  the  same 
reciprocity,  as  regards  solid  bodies,  has  been  variously 
illustrated;  while  the  labours,  theoretical  and  ex- 
perimental, of  Kirchhoff  have  given  this  subject  a 
wonderful  expansion,  and  enriched  it  by  applications 
of  the  highest  kind.  To  their  results  are  now  to 
be  added  the  foregoing,  whereby  gases  and  vapours, 
which  have  been  hitherto  thought  inaccessible  to  ex- 


*  See  page  15  for  a  definition  of  vis  viva. 

*  When  heated  air  imparts  its  motion  to  another  gas  or  vapour, 
the  transference  of  heat  is  accompanied  by  a  change  of  vibrating 
period.  The  Dynamic  Radiation  of  vapours  ia  rendered  possible  by 
this  transmutation  of  vibrations. 


RADIATION.  65 

periments  with  the  thermo-electric  pile,  are  proved  by 
it  to  exhibit  the  indissoluble  duality  of  radiation  and 
absorption,  the  influence  of  chemical  combination  on 
both  being  exhibited  in  the  most  decisive  and  extra- 
ordinary way. 


15.  Influence  of   Vibrating  Period  and  Molecluai 
Form,     Physical  Analysis  of  the  Human  Breath. 

In  the  foregoing  experiments  with  gases  and  vapours 
we  have  employed  throughout  invisible  rays,  and  found 
some  of  these  bodies  so  impervious  to  radiant  heat,  that 
in  lengths  of  a  few  feet  they  intercept  every  ray  as 
effectually  as  a  layer  of  pitch.  The  substances,  how- 
ever, which  show  themselves  thus  opaque  to  radiant  heat 
are  perfectly  transparent  to  light.  Now  the  rays  of 
light  differ  from  those  of  invisible  heat  merely  in  point 
of  period,  the  former  failing  to  affect  the  retina  because 
their  periods  of  recurrence  are  too  slow.  Hence,  in 
some  way  or  other,  the  transparency  of  our  gases  and 
vapours  depends  upon  the  periods  of  the  waves  which 
impinge  upon  them.  What  is  the  nature  of  this  depend- 
ence ?  The  admirable  researches  of  Kirchhoff  help  us 
to  an  answer.  The  atoms  and  molecules  of  every  gas 
have  certain  definite  rates  of  oscillation,  and  those  waves 
of  ether  are  most  copiously  absorbed  whose  periods 
of  recurrence  synchronise  with  those  of  the  atomic 
groups  amongst  which  they  pass.  Thus,  when  we  find 
the  invisible  rays  absorbed  and  the  visible  ones  trans- 
mitted by  a  layer  of  gas,  we  conclude  that  the  oscillating 
periods  of  the  atoms  constituting  the  gaseous  molecules 
coincide  with  those  of  the  invisible,  and  not  with  those 
of  the  visible  spectrum. 


66  FEAGMENTS  OF  SCIENCE. 

It  requires  some  discipline  of  the  imagiDation  to 
form  a  clear  picture  of  this  process.  Such  a  picture  is, 
however,  possible,  and  ought  to  be  obtained.  When 
the  waves  of  ether  impinge  upon  molecules  whose 
periods  of  vibration  coincide  with  the  recurrence  of  the 
undulations,  the  timed  strokes  of  the  waves  augment 
the  vibration  of  the  molecules,  as  a  heavy  pendulum  is 
set  in  motion  by  well-timed  puffs  of  breath.  Millions 
of  millions  of  shocks  are  received  every  second  from  the 
calorific  waves ;  and  it  is  not  difficult  to  see  that  aa 
every  wave  arrives  just  in  time  to  repeat  the  action  of 
its  predecessor,  the  molecules  must  finally  be  caused  to 
swing  through  wider  spaces  than  if  the  arrivals  were  not 
so  timed.  In  fact,  it  is  not  difficult  to  see  that  an 
assemblage  of  molecules,  operated  upon  by  contending 
waves,  might  remain  practically  quiescent.  This  is 
actually  the  case  when  the  waves  of  the  visible  spectrum 
pass  through  a  transparent  gas  or  vapour.  There  is 
here  no  sensible  transference  of  motion  from  the  ether 
to  the  molecules ;  in  other  words,  there  is  no  sensible 
absorption  of  heat. 

One  striking  example  of  the  influence  of  period  may 
be  here  recorded.  Carbonic  acid  gas  is  one  of  the  feeblest 
absorbers  of  the  radiant  heat  emitted  by  solid  bodies. 
It  is,  for  example,  to  a  great  extent  transparent  to  the 
rays  emitted  by  the  heated  copper  plate  already  referred 
to.  There  are,  however,  certain  rays,  comparatively 
few  in  number,  emitted  by  the  copper,  to  which  the 
carbonic  acid  is  impervious;  and  could  we  obtain  a 
source  of  heat  emitting  such  rays  only,  we  should  find 
carbonic  acid  more  opaque  to  the  radiation  from  that 
source,  than  any  other  gas.  Such  a  source  is  actually 
found  in  the  flame  of  carbonic  oxide,  where  hot  carbonic 
acid  constitutes  the  main  radiating  body.  Of  the  rays 
emitted  by  our  heated  plate  of  copper,  defiant  gas  absorbs 


RADIATION.  67 

ten  times  the  quantity  absorbed  by  carbonic  acid.  Of 
the  rays  emitted  by  a  carbonic  oxide  flame,  carbonic 
acid  absorbs  twice  as  much  as  defiant  gas.  This  won- 
derful change  in  the  power  of  the  former,  as  an  absorber, 
is  simply  due  to  the  fact,  that  the  periods  of  the  hot  and 
cold  carbonic  acid  are  identical,  and  that  the  waves  from 
the  flame  freely  transfer  their  motion  to  the  molecules 
which  synchronise  with  them.  Thus  it  is  that  the  tenth 
of  an  atmosphere  of  carbonic  acid,  enclosed  in  a  tube 
four  feet  long,  absorbs  60  per  cent,  of  the  radiation  from 
a  carbonic  oxide  flame,  while  one-thirtieth  of  an  atmo- 
sphere absorbs  48  per  cent,  of  the  heat  from  the  same 
source. 

In  fact,  the  presence  of  the  minutest  quantity  of  car- 
bonic acid  may  be  detected  by  its  action  on  the  rays 
from  the  carbonic  oxide  flame.  Carrying,  for  example, 
the  dried  human  breath  into  a  tube  four  feet  long,  the 
absorption  there  effected  by  the  carbonic  acid  of  the 
breath  amounts  to  50  per  cent,  of  the  entire  radiation. 
Eadiant  heat  may  indeed  be  employed  as  a  means  of 
determining  practically  the  amount  of  carbonic  acid 
expired  from  the  lungs.  My  late  assistant,  Mr.  Barrett, 
while  under  my  direction,  made  this  determination. 
The  absorption  produced  by  the  breath  freed  from  its 
moisture,  but  retaining  its  carboDic  acid,  was  first  deter- 
mined. Carbonic  acid,  artificially  prepared,  was  then 
mixed  with  dry  air  in  such  proportions  that  the  action 
of  the  mixture  upon  the  rays  of  heat  was  the  same 
as  that  of  the  dried  breath.  The  percentage  of  the 
former  being  known,  immediately  gave  that  of  the 
latter.  The  same  breath,  analysed  chemically  by  Dr. 
Frankland,  and  physically  by  Mr.  Barrett,  gave  the 
following  results : — 


68  FEAGMENTS  0?  SCIENCE. 

Percentage  of  Carbonic  Add  in  the  Human  Breath, 

Chemical  analysis  Physical  analysk 

4-66 4-56 

5-33 6-22 

It  is  thus  proved  that  in  the  quantity  of  ethereal 
motion  which  it  is  competent  to  take  up,  we  have  a  prac- 
tical measure  of  the  carbonic  acid  of  the  breath,  and 
hence  of  the  combustion  going  on  in  the  human  lungs. 

Still  this  question  of  period,  though  of  the  utmost 
importance,  is  not  competent  to  account  for  the  whole 
of  the  observed  facts.  The  ether,  as  far  as  we  know, 
accepts  vibrations  of  all  periods  with  the  same  readiness. 
To  it  the  oscillations  of  an  atom  of  free  oxygen  are  just 
as  acceptable  as  those  of  the  atoms  in  a  molecule  of 
oletiant  gas  ;  that  the  vibrating  oxygen  then  stands  so 
far  below  the  defiant  gas  in  radiant  power  must  be  re- 
ferred not  to.  period,  but  to  some  other  peculiarity.  The 
atomic  group  which  constitutes  the  molecule  of  olefiant 
gas,  produces  many  thousand  times  the  disturbance 
caused  by  the  oxygen,  it  may  be  because  the  group  is 
able  to  lay  a  vastly  more  powerful  hold  upon  the  ether 
than  the  single  atoms  can.  Another,  and  probably  very 
potent  cause  of  the  difference  may  be,  that  the  vibra- 
tions, being  those  of  the  constituent  atoms  of  the  mole- 
cule,^ are  generated  in  highly  condensed  ether,  which 
acts  like  condensed  air  upon  sound.  But  whatever  may 
be  the  fate  of  these  attempts  to  visualise  the  physics  of 
the  process,  it  will  still  remain  true,  that  to  account  for 
the  phenomena  of  radiation  and  absorption  we  must 
take  into  consideration  the  shape,  size,  and  condition  of 
the  ether  within  the  molecules,  by  which  the  external 
ether  is  disturbed. 

•  See  '  Pbysicali  Considerations,*  Art.  iv.  p.  102. 


RADIATION.  69 


16.  Summary  and  Concluaioii, 

Let  us  now  cast  a  momentary  glance  over  the  ground 
that  we  have  left  behind.  The  general  nature  of  light 
and  heat  was  first  briefly  described  :  the  compounding  of 
matter  from  elementary  atoms,  and  the  influence  of  the 
act  of  combination  on  radiation  and  absorption,  were 
considered  and  experimentally  illustrated.  Through  the 
transparent  elementary  gases  radiant  heat  was  found  to 
pass  as  through  a  vacuum,  while  many  of  the  compound 
gases  presented  almost  impassable  obstacles  to  the  calor- 
ific waves.  This  deportment  of  the  simple  gases  directed 
our  attention  to  other  elementary  bodies,  the  examina- 
tion of  which  led  to  the  discovery  that  the  element  iodine, 
dissolved  in  bisulphide  of  carbon,  possesses  the  power 
of  detaching,  with  extraordinary  sharpness,  the  light  of 
the  spectrum  from  its  heat,  intercepting  all  luminous 
rays  up  to  the  extreme  red,  and  permitting  the  calorific 
rays  beyond  the  red  to  pass  freely  through  it.  This  sub- 
stance was  then  employed  to  filter  the  betims  of  the 
electric  light,  and  to  form  foci  of  invisible  rays  so  in- 
tense as  to  produce  almost  all  the  effects  obtainable  in 
an  ordinary  fire.  Combustible  bodies  were  burnt,  and 
refractory  ones  were  raised  to  a  white  heat,  by  the  con- 
centrated invisible  rays.  Thus,  by  exalting  their  re- 
frangibility,  the  invisible  rays  of  the  electric  light  were 
rendered  visible,  and  all  the  colours  of  the  solar  spectrum 
were  extracted  from  utter  darkness.  The  extreme  rich- 
ness of  the  electric  light  in  invisible  rays  of  low  re- 
frangibility  was  demonstrated,  one-eighth  only  of  its 
radiation  consisting  of  luminous  rays.  The  deadness  of 
the  optic  nerve  to  those  invisible  rays  was  proved,  and 
experiments  were  then  added  to  show  that  the  bright 
and  the  dark  rays  of  a  solid  body,  raised  gradually  to 


70  FEAGMENTS  OF  SCIENCK 

incandescence,  are  strengthened  together;  intense 
dark  heat  being  an  invariable  accompaniment  of  in- 
tense white  heat.  A  sun  could  not  be  formed,  or  a 
meteorite  rendered  luminous,  on  any  other  condition. 
The  light-giving  rays  constituting  only  a  small  fraction 
of  the  total  radiation,  their  unspeakable  importance  to 
us  is  due  to  the  fact,  that  their  periods  are  attuned  to 
the  special  requirements  of  the  eye. 

Among  the  vapours  of  volatile  liquids  vast  differ- 
ences were  also  found  to  exist,  as  regards  their  powers 
of  absorption.  We  followed  various  molecules  from  a 
state  of  liquid  to  a  state  of  gas,  and  found,  in  both 
states  of  aggregation,  the  power  of  the  individual  mole- 
cules equally  asserted.  The  position  of  a  vapour  as  an 
absorber  of  radiant  heat  was  shown  to  be  determined 
by  that  of  the  liquid  from  which  it  is  derived.  Ee- 
versing  our  conceptions,  and  regarding  the  molecules 
of  gases  and  vapours  not  as  the  recipients  but  as  the 
originators  of  wave-motion;  not  as  absorbers  but  as 
radiators  ;  it  was  proved  that  the  powers  of  absorption 
and  radiation  went  hand  in  hand,  the  self-same  chemical 
act  which  rendered  a  body  competent  to  intercept  the 
waves  of  ether,  rendering  it  competent,  in  the  same 
degree,  to  generate  them.  Perfumes  were  next  sub- 
jected to  examination,  and,  notwithstanding  their 
extraordinary  tenuity,  they  were  found  vastly  superior, 
in  point  of  absorptive  power,  to  the  body  of  the  air  in 
which  they  were  diffused.  We  were  led  thus  slowly  up 
to  the  examination  of  the  most  widely  diffused  and 
most  important  of  all  vapours — the  aqueous  vapour  of 
our  atmosphere,  and  we  found  in  it  a  potent  absorber 
of  the  purely  calorific  rays.  The  power  of  this  sub- 
stance to  influence  climate,  and  its  general  influence 
on  the  temperature  of  the  earth,  were  then  briefly 
dwelt   upon.      A   cobweb   spread   above   a  blossom  ia 


RADIATION.  71 

sufficient  to  protect  it  from  nightly  chill;  and  thua 
the  aqueous  vapour  of  our  air,  attenuated  as  it  is,  checks 
the  drain  of  terrestrial  heat,  and  saves  the  surface  of 
our  planet  from  the  refrigeration  which  would  assuredly 
accrue,  were  no  such  substance  interposed  between 
it  and  the  voids  of  space.  We  considered  the  influence 
of  vibratiug  period,  and  molecular  form,  on  absorption 
and  radiation,  and  finally  deduced,  from  its  action 
upon  radiant  heat,  the  exact  amount  of  carbonic  acid 
expired  by  the  human  lungs. 

Thus,  in  brief  outline,  were  placed  before  you  some 
of  the  results  of  recent  enquiries  in  the  domain  of 
Kadiation,  and  my  aim  throughout  has  been  to  raise  in 
your  minds  distinct  physical  images  of  the  various  pro- 
cesses involved  in  our  researches.  It  is  thought  by 
some  that  natural  science  has  a  deadening  influence  on 
the  imagination,  and  a  doubt  might  fairly  be  raised  as 
to  the  value  of  any  study  which  would  necessarily  have 
this  effect.  But  the  experience  of  the  last  hour  must, 
I  think,  have  convinced  you,  that  the  study  of  natural 
science  goes  hand  in  hand  with  the  culture  of  ^  the  ima- 
gination. Throughout  the  greater  part  of  this  discourse 
we  have  been  sustained  by  this  faculty.  We  have  been 
picturing  atoms,  and  molecules,  and  vibrations,  and 
waves,  which  eye  has  never  seen  nor  ear  heard,  and 
which  can  only  be  discerned  by  the  exercise  of  ima- 
gination. This,  in  fact,  is  the  faculty  which  enables  us 
to  transcend  the  boundaries  of  sense,  and  connect  the 
phenomena  of  our  visible  world  with  those  of  an  in- 
visible one.  Without  imagination  we  never  could  have 
risen  to  the  conceptions  which  have  occupied  us  here 
to-day ;  and  in  proportion  to  your  power  of  exercising 
this  faculty  aright,  and  of  associating  definite  mental 
images  with  the  terms  employed,  will  be  the  pleasure 
and  the  profit  which  you  will  derive  from  this  lecture. 
6 


72  FEAGMENTS  OF  SCIENCE. 

The  outward  facts  of  nature  are  insufficient  to  satisfy 
the  mind.  We  cannot  be  content  with  knowing  that 
the  light  and  heat  of  the  sun  iUiiminate  and  warm  the 
world.  We  are  led  irresistibly  to  enquire,  '  What  is 
light,  and  what  is  heat  ? '  and  this  question  leads  us  at 
once  out  of  the  region  of  sense  into  that  of  imagination.^ 
Thus  pondering,  and  questioning,  and  striving  to 
supplement .  that  which  is  felt  and  seen,  but  which  is 
incomplete,  by  something  unfelt  and  unseen  which  is 
necessary  to  its  completeness,  men  of  genius  have  in 
part  discerned,  not  only  the  nature  of  light  and  heat, 
but  also,  through  them,  the  general  relationship  of 
natural  phenomena.  The  working  power  of  Nature 
consists  of  actual  or  potential  motion,  of  which  all 
its  phenomena  are  but  special  forms.  This  motion 
manifests  itself  in  tangible  and  in  intangible  matter, 
being  incessantly  transferred  from  the  one  to  the  other, 
and  incessantly  transformed  by  the  change.  It  is  as 
real  in  the  waves  of  the  ether  as  in  the  waves  of  the 
sea ;  the  latter — derived  as  they  are  from  winds,  which 
in  their  turn  are  derived  from  the  sun — are,  indeed, 
nothing  more  than  the  heaped-up  motion  of  the  ether 
waves.  It  is  the  calorific  waves  emitted  by  the  sun 
which  heat  our  air,  produce  our  winds,  and  hence 
agitate  our  ocean.  And  whether  they  break  in  foam 
upon  the  shore,  or  rub  silently  against  the  ocean's  bed, 
or  subside  by  the  mutual  friction  of  their  own  parts, 
the  sea  waves,  which  cannot  subside  without  producing 
heat,  finally  resolve  themselves  into  waves  of  ether, 
thus  regenerating  the  motion  from  which  their  tempo- 
rary existence  was  derived.  This  connection  is  typical. 
Nature  is  not  an  aggregate  of  independent  parts,  but 
an  organic  whole.     If  you  open  a  piano  and  sing  into 

»  This  line  of  thought  was  pursued  further  five  years  subsc- 
queutly.     See  *  Scientific  Use  of  the  Imagination  '  in  Vol.  IL 


RADIATION.  -73 

it,  a  certain  string  will  respond.  Change  the  pitch  of 
your  voice ;  the  first  string  ceases  to  vibrate,  but  another 
replies.  Change  again  the  pitch ;  the  first  two  strings 
are  silent,  while  another  resounds.  Thus  is  sentient 
man  acted  on  by  Nature,  the  optic,  the  auditory,  and 
other  nerves  of  the  human  body  being  so  many  strings 
differently  tuned,  and  responsive  to  different  forms  of 
the  universal  power. 


III. 

ON  RADIANT  HEAT  IN  RELATION  TO  THE 
COLOUR  AND  CHEMICAL  CONSTITUTION  OF 
BODIES.^ 

ONE  of  the  most  important  functions  of  physical 
science,  considered  as  a  discipline  of  the  mind,  is 
to  enable  us  by  means  of  the  sensible  processes  of  Nature 
to  apprehend  the  insensible.  The  sensible  processes 
give  direction  to  the  line  of  thought;  but  this  once 
given,  the  length  of  the  line  is  not  limited  by  the 
boundaries  of  the  senses.  Indeed,  the  domain  of  the 
senses,  in  Nature,  is  almost  infinitely  small  in  com- 
parison with  the  vast  region  accessible  to  thought  which 
lies  beyond  them.  From  a  few  observations  of  a  comet, 
when  it  comes  within  the  range  of  his  telescope,  an 
astronomer  can  calculate  its  path  in  regions  which  no 
telescope  can  reach  :  and  in  like  manner,  by  means  of 
data  furnished  in  the  narrow  world  of  the  senses,  we 
make  ourselves  at  home  in  other  and  wider  worlds, 
which  are  traversed  by  the  intellect  alone. 

From  the  earliest  ages  the  questions,  'Wliat  is 
light?'  and  *What  is  heat?'  have  occurred  to  the 
minds  of  men ;  but  these  questions  never  would  have 
been  answered  had  they  not  been  preceded  by  the  ques- 
tion, '  What  is  sound  ? '  Amid  the  grosser  phenomena 
of  acoustics  the  mind  was  first  disciplined,  conceptions 

•  A  discourse  delivered  in  the  Kojral  Institution  of  Grejif 
Pritain,  Jan.  19,  1866. 


RADIANT  HEAT  AND  ITS  EELATIONS.  75 

being  thus  obtained  from  direct  observation,  which 
were  afterwards  applied  to  phenomena  of  a  character 
far  too  subtle  to  be  observed  directly.  Sound  we  know 
to  be  due  to  vibratory  motion.  A  vibrating  tuning- 
fork,  for  example,  moulds  the  air  around  it  into  im- 
dulations  or  waves,  which  speed  away  on  all  sides  with 
a  certain  measured  velocity,  impinge  upon  the  drum  of 
the  ear,  shake  the  auditory  nerve,  and  awake  in  the 
brain  the  sensation  of  sound.  When  sufficiently  near 
a  sounding  body  we  can  feel  the  vibrations  of  the  air. 
A  deaf  man,  for  example,  plunging  his  band  into  a 
bell  when  it  is  sounded,  feels  through  the  common 
nerves  of  his  body  those  tremors  which,  when  imparted 
to  the  nerves  of  healthy  ears,  are  translated  into  sound. 
There  are  various  ways  of  rendering  those  sonorous 
vibrations  not  only  tangible  but  visible;  and  it  was 
not  until  numberless  experiments  of  this  kind  had  been 
executed,  that  the  scientific  investigator  abandoned 
himself  wholly,  and  without  a  shadow  of  misgiviog,  to 
the  conviction  that  what  is  sound  within  us  is,  outside 
of  us,  a  motion  of  the  air. 

But  once  having  established  this  fact — once  having 
proved  beyond  all  doubt  that  the  sensation  of  sound  is 
produced  by  an  agitation  of  the  auditory  nerve — the 
thought  soon  suggested  itself  that  light  might  be  due 
to  an  agitation  of  the  optic  nerve.  This  was  a  great 
step  in  advance  of  that  ancient  notion  which  regarded 
light  as  something  emitted  by  the  eye,  and  not  as  any- 
thing imparted  to  it.  But  if  light  be  produced  by  an 
agitation  of  the  retina,  what  is  it  that  produces  the 
agitation?  Newton,  you  know,  supposed  minute 
particles  to  be  shot  through  the  humours  of  the  eye 
against  the  retina,  which  he  supposed  to  hang  like  a 
target  at  the  back  of  the  eye.  The  impact  of  these 
particles  against   the   target,  Newton   believed  to  be 


76  FEAGMENTS  OF  SCIENCE. 

the  cause  of  light.  But  Newton's  notion  has  not 
held  its  ground,  being  entirely  driven  from  the  field 
by  the  more  wonderful  and  far  more  philosophical 
notion  that  light,  like  sound,  is  a  product  of  wave- 
motion. 

The  domain  in  which  this  motion  of  light  is  carried 
on  lies  entirely  beyond  the  reach  of  our  senses.  The 
waves  of  light  require  a  medium  for  their  formation 
and  propagation ;  but  we  cannot  see,  or  feel,  or  taste, 
or  smell  this  medium.  How,  then,  has  its  existence 
been  established  ?  By  showing,  that  by  the  assump- 
tion of  this  wonderful  intangible  ethar^  all  the  pheno- 
mena of  optics  are  accounted  for,  with  a  fulness,  and 
clearness,  and  conclusiveness,  which  leave  no  desire  of 
the  intellect  unsatisfied.  When  the  law  of  gravitation 
first  suggested  itself  to  the  mind  of  Newton,  what  did 
he  do  ?  He  set  himself  to  examine  whether  it  accounted 
for  all  the  facts.  He  determined  the  courses  of  the 
planets ;  he  calculated  the  rapidity  of  the  moon's  fall 
towards  the  earth ;  he  considered  the  precession  of  the 
equinoxes,  the  ebb  and  flow  of  the  tides,  and  found  all 
explained  by  the  law  of  gravitation.  He  therefore 
regarded  this  law  as  established,  and  the  verdict  of 
science  subsequently  confirmed  his  conclusion.  On 
similar,  and,  if  possible,  on  stronger  grounds,  we  found 
our  belief  in  the  existence  of  the  universal  ether.  It 
explains  facts  far  more  various  and  complicated  than 
those  on  which  Newton  based  his  law.  If  a  single 
phenomenon  could  be  pointed  out  which  the  ether  is 
proved  incompetent  to  explain,  we  should  have  to  give 
it  up ;  but  no  such  phenomenon  has  ever  been  pointed 
out.  It  is,  therefore,  at  least  as  certain  that  space  is 
filled  with  a  medium,  by  means  of  which  suns  and  stars 
diffuse  theli  radiant  power,  as  that  it  is  traversed  by 
that   force   wnich    holds   in   its    grasp,  not   only   our 


RADIANT  HEAT  AND   ITS  KELATIONS.  77 

planetary  system,  but  the  immeasurable  heavens  them- 
selves. 

There  is  no  more  wonderful  instance  than  this  of 
the  production  of  a  line  of  thought,  from  the  world  of 
the  senses  into  the  region  of  pure  imagination.  I 
mean  by  imagination  here,  not  that  play  of  fancy  which 
can  give  to  airy  nothings  a  local  habitation  and  a  name, 
but  that  power  which  enables  the  mind  to  conceive 
realities  which  lie  beyond  the  range  of  the  senses — to 
present  to  itself  distinct  images  of  processes  which, 
though  miglity  in  the  aggregate  beyond  all  conception, 
are  so  minute  individually  as  to  elude  all  observation. 
It  is  the  waves  of  air  excited  by  a  tuning-fork  which 
render  its  vibrations  audible.  It  is  the  waves  of  ether 
sent  forth  from  those  lamps  overhead  which  render  them 
luminous  to  us  ;  but  so  minute  are  these  waves,  that 
it  would  take  from  30,000  to  60,000  of  them  placed 
end  to  end  to  cover  a  single  inch.  Their  number,  how- 
ever, compensates  for  their  minuteness.  Trillions  of  them 
have  entered  your  eyes,  and  hit  the  retina  at  the  backs 
of  your  eyes,  in  the  time  consumed  in  the  utterance 
of  the  shortest  sentence  of  this  discourse.  This  is  the 
steadfast  result  of  modern  research  ;  but  we  never  could 
have  reached  it  without  previous  discipline.  We  never 
could  have  measured  the  waves  of  light,  nor  even 
imagined  them  to  exist,  had  we  not  previously  exercised 
ourselves  among  the  waves  of  sound.  Sound  and  light 
are  now  mutually  helpful,  the  conceptions  of  each  being 
expanded,  strengthened,  and  defined  by  the  conceptions 
of  the  other. 

The  ether  which  conveys  the  pulses  of  light  and 
heat  not  only  fills  celestial  space,  swathing  suns,  and 
planets,  and  moons,  but  it  also  encircles  the  atoms  of 
which  these  bodies  are  composed.  It  is  the  motion  of 
these   atoms,    and  not  that   of  any  sensible  parts  of 


78  FEA.GMENTS  OF  SCIENCE. 

bodies,  that  the  ether  conveys.  This  motion  is  the 
objective  cause  of  what,  in  our  sensations,  are  light  and 
heat.  An  atom,' then,  sending  its  pulses  through  the 
ether,  resembles  a  tuning-fork  sending  its  pulses 
through  the  air.  Let  us  look  for  a  moment  at  this 
thrilling  medium,  and  briefly  consider  its  relation  to 
the  bodies  whose  vibrations  it  conveys.  Different  bodies, 
when  heated  to  the  same  temperature,  possess  very  dif- 
ferent powers  of  agitating  the  ether :  some  are  good 
radiators,  others  are  bad  radiators  ;  which  means  that 
some  are  so  constituted  as  to  communicate  their  atomic 
motion  freely  to  the  ether,  producing  therein  powerful 
undulations  ;  while  the  atoms  of  others  are  unable  thus 
to  communicate  their  motions,  but  glide  through  the 
medium  without  materially  disturbing  its  repose.  Recent 
experiments  have  proved  that  elementary  bodies,  except 
under  certain  anomalous  conditions,  belong  to  the  class 
of  bad  radiators.  An  atom,  vibrating  in  the  ether,  re- 
sembles a  naked  tuning-fork  vibrating  in  the  air.  The 
amount  of  motion  communicated  to  the  air  by  the  thin 
prongs  is  too  small  to  evoke  at  any  distance  the  sensa- 
tion of  sound.  But  if  we  permit  the  atoms  to  com- 
bine chemically  and  form  molecules,  the  result,  in 
many  cases,  is  an  enormous  change  in  the  power  of 
radiation.  The  amount  of  ethereal  disturbance,  pro- 
duced by  the  combined  atoms  of  a  body,  may  be  many 
thousand  times  that  produced  by  the  same  atoms  when 
uncombined. 

The  pitch  of  a  musical  note  depends  upon  the 
rapidity  of  its  vibrations,  or,  in  other  words,  on  the 
length  of  its  waves.  Now,  the  pitch  of  a  note  answers 
to  the  colour  of  light.  Taking  a  slice  of  white  light 
from  the  sun,  or  from  an  electric  lamp,  and  causing  the 
light  to  pass  through  an  arrangement  of  prisms,  it  is 
decomposed.     We  have  the  effect  obtained  by  Newton, 


RADIANT  HEAT  AND  ITS  RELATIONS  79 

who  first  unrolled  the  solar  beam  into  the  splendours  of 
the  solar  spectrum.  At  one  end  of  this  spectrum  we 
Lave  red  light,  at  the  other,  violet ;  and  between  those 
extremes  lie  the  other  prismatic  colours.  As  we  advance 
along  the  spectrum  from  the  red  to  the  violet,  the 
pitch  of  the  light — if  I  may  use  the  expression — 
heightens,  the  sensation  of  violet  being  produced  b}^ 
a  more  rapid  succession  of  impulses  than  that  which 
produces  the  impression  of  red.  The  vibrations  of  the 
violet  are  about  twice  as  rapid  as  those  of  the  red ;  in 
other  words,  the  range  of  the  visible  spectrum  is  about 
an  octave. 

There  is  no  solution  of  continuity  in  this  spectrum  ; 
one  colour  changes  into  another  by  insensible  gradations. 
It  is  as  if  an  infinite  number  of  tuning-forks,  of  gradu- 
ally augmenting  pitch,  were  vibrating  at  the  same  time. 
But  turning  to  another  spectrum — that,  namely,  ob- 
tained from  the  incandescent  vapour  of  silver — you 
observe  that  it  consists  of  two  narrow  and  intensely 
luminous  green  bands.  Here  it  is  as  if  two  forks  only, 
of  slightly  different  pitch,  were  vibrating.  The  length 
of  the  waves  which  produce  this  first  band  is  such  that 
47,460  of  them,  placed  end  to  end,  would  fill  an  inch. 
The  waves  which  produce  the  second  band  are  a  little 
shorter;  it  would  take  of  these  47,920  to  fill  an  inch. 
In  the  case  of  the  first  band,  the  number  of  impulses 
imparted,  in  one  second,  to  every  eye  which  sees  it,  is 
577  millions  of  millions  ;  while  the  number  of  impulses 
imparted,  in  the  same  time,  by  the  second  band  is  600 
millions  of  millions.  We  may  project  upon  a  white 
screen  the  beautiful  stream  of  green  light  from  which 
these  bands  were  derived.  This  luminous  stream  is  the 
incandescent  vapour  of  silver.  The  rates  of  vibration 
of  the  atoms  of  that  vapour  are  as  rigidly  fixed  as  those 
of  two  tuning-forks  ;  and  to  whatever  height  the  tern- 


80  FRAGMENTS  OF  SCIENCE. 

perature  of  the  vapour  may  be  raised,  the  rapidity  ol 
its  vibrations,  and  consequently  its  colour,  which  wholly 
depends  upon  that  rapidity,  remain  unchanged. 

The  vapoui  of  water,  as  well  as  the  vapour  of  silver, 
has  its  definite  periods  of  vibration,  and  these  are  such 
as  to  disqualify  the  vapour,  when  acting  freely  as  such, 
from  being  raised  to  a  white  heat.  The  oxyhydrogen 
flame,  for  example,  consists  of  hot  aqueous  vapour.  It 
is  scarcely  visible  in  the  air  of  this  room,  and  it  would 
be  still  less  visible  if  we  could  burn  the  gas  in  a  clean 
atmosphere.  But  the  atmosphere,  even  at  the  summit 
of  Mont  Blanc,  is  dirty ;  in  London  it  is  more  than 
dirty;  and  the  burning  dirt  gives  to  this  flame  the 
greater  portion  of  its  present  light.  But  the  heat  of 
the  flame  is  enormous.  Cast  iron  fuses  at  a  tempera- 
ture of  2,000°  Fahr. ;  while  the  temperature  of  the 
oxyhydrogen  flame  is  6,000°  Fahr.  A  piece  of  platinum 
is  heated  to  vivid  redness,  at  a  distance  of  two  inches 
beyond  the  visible  termination  of  the  flame.  The 
vapour  which  produces  incandescence  is  here  absolutely 
dark.  In  the  flame  itself  the  platinum  is  raised  to 
dazzling  whiteness,  and  is  even  pierced  by  the  flame. 
When  this  flame  impinges  on  a  piece  of  lime,  we  have 
the  dazzling  Drummond  light.  But  the  light  is  here 
due  to  the  fact  that  when  it  impinges  upon  the  solid 
body,  the  vibrations  excited  in  that  body  by  the  flame 
are  of  periods  different  from  its  own. 

Thus  far  we  have  fixed  our  attention  on  atoms  and 
molecules  in  a  state  of  vibration,  and  surrounded  by  a 
medium  which  accepts  their  vibrations,  and  transmits 
them  through  space.  Ikit  suppose  the  waves  generated 
by  one  system  of  molecules  to  impinge  upon  another 
system,  how  will  the  waves  be  affected  ?  Will  they  be 
stopped,  or  will  they  be  permitted  to  pass  ?  Will  they 
transfer  their  motion  to  the  molecules  on  which  they 


RADIANT  HEAT  AND   ITS  RELATIONS.  81 

impinge,  or  will  they  glide  round  the  molecules,  through 
the  intermolecular  spaces,  and  thus  escape  ? 

The  answer  to  this  question  depends  upon  a  condi- 
tion which  may  be  beautifully  exemplified  by  an  experi- 
ment on  sound.  These  two  tuning-forks  are  tuned 
absolutely  alike.  They  vibrate  with  the  same  rapidity, 
and,  mounted  thus  upon  their  resonant  cases,  you  hear 
them  loudly  sounding  the  same  musical  note.  Si/opping 
one  of  the  forks,  I  throw  the  other  into  strong  vibration, 
and  bring  that  other  near  the  silent  fork,  but  not  into 
contact  with  it.  Allowing  them  to  continue  in  this 
position  for  four  or  five  seconds,  and  then  stopping  the 
vibrating  fork,  the  sound  does  not  cease.  The  second 
fork  has  taken  up  the  vibrations  of  its  neighbour,  and 
is  now  sounding  in  its  turn.  Dismounting  one  of  the 
forks,  and  permitting  the  other  to  remain  upon  its 
stand,  I  throw  the  dismounted  fork  into  strong  vibra- 
tion. You  cannot  hear  it  sound.  Detached  from  its 
case,  the  amount  of  motion  which  it  can  communicate 
to  the  air  is  too  small  to  be  sensible  at  any  distance. 
When  the  dismounted  fork  is  brought  close  to  the 
mounted  one,  but  not  into  actual  contact  with  it,  out  of 
the  silence  rises  a  mellow  sound.  Whence  comes  it  ? 
From  the  vibrations  which  have  been  transferred  from 
the  dismounted  fork  to  the  mounted  one. 

That  the  motion  should  thus  transfer  itself  through 
the  air  it  is  necessary  that  the  two  forks  should  be  in 
perfect  unison.  If  a  morsel  of  wax  not  larger  than  a 
pea  be  placed  on  one  of  the  forks,  it  is  rendered  thereby 
powerless  to  affect,  or  to  be  affected  by,  the  other.  It 
is  easy  to  understand  this  experiment.  The  pulses  of 
the  one  fork  can  affect  the  other,  because  they  are  per- 
fectly timed.  A  single  pulse  causes  the  prong  of  the 
silent  fork  to  vibrate  through  an  infinitesimal  space. 
But  just   as   it   has   completed   this  small   vibration. 


82  FKAGMENTS  OF  SCIENCB. 

another  pulse  is  ready  to  strike  it.  Thus,  the  impulses 
add  themselves  together.  In  the  five  seconds  during 
which  the  forks  were  held  near  each  other,  the  vibrating 
fork  sent  1,280  waves  against  its  neighbour  and  those 
1,280  shocks,  all  delivered  at  the  proper  moment,  all,  as 
I  have  said,  perfectly  timed,  have  given  such  strength 
to  the  vibrations  of  the  mounted  fork  as  to  render  them 
audible  to  all. 

Another  curious  illustration  of  the  influence  of 
synchronism  on  musical  vibrations,  is  this:  Three  small 
gas-flames  are  inserted  into  three  glass  tubes  of  different 
lengths.  Each  of  these  flames  can  be  caused  to  emit 
a  musical  note,  the  pitch  of  which  is  determined  by 
the  length  of  the  tube  surrounding  the  flame.  The 
shorter  the  tube  the  higher  is  the  pitch.  The  flames 
are  now  silent  within  their  respective  tubes,  but  each 
of  them  can  be  caused  to  respond  to  a  proper  note 
sounded  anywhere  in  this  room.  With  an  instrument 
called  a  syren,  a  powerful  musical  note,  of  gradually 
increasing  pitch,  can  be  produced.  Beginning  with  a 
low  note,  and  ascending  gradually  to  a  higher  one,  we 
finally  attain  the  pitch  of  the  flame  in  the  longest  tube. 
The  moment  it  is  reached,  the  flame  bursts  into  song. 
The  other  flames  are  still  silent  within  their  tubes. 
But  by  urging  the  instrument  on  to  higher  notes,  the 
second  flame  is  started,  and  the  third  alone  remains. 
A  still  higher  note  starts  it  also.  Thus,  as  the  sound  of 
the  syren  rises  gradually  in  pitch,  it  awakens  every 
flame  in  passing,  by  striking  it  with  a  series  of  waves 
whose  periods  of  recurrence  are  similar  to  its  own. 

Now  the  wave-motion  from  the  syren  is  in  part  taken 
up  by  the  flame  which  synchronises  with  the  waves ;  and 
were  these  waves  to  impinge  upon  a  multitude  of  flames, 
instead  of  upon  one  flame  only,  the  transference  might 
be  so  great  as  to  absorb  the  whole  of  the  original  wave 


RADIANT  HEAT  AKD  ITS  RELATIONS.  83 

motion.  Let  us  apply  these  facts  to  radiant  heat.  This 
blue  flame  is  the  flame  of  carbonic  oxide ;  this  trans- 
parent gas  is  carbonic  acid  gas.  In  the  blue  flame  we 
have  carbonic  acid  intensely  heated,  or,  in  other  words, 
in  a  state  of  intense  vibration.  It  thus  resembles  the 
sounding  fork,  while  this  cold  carbonic  acid  resembles 
the  silent  one.  What  is  the  consequence  ?  Through 
the  synchronism  of  the  hot  and  cold  gas,  the  waves 
emitted  by  the  former  are  intercepted  by  the  latter, 
the  transmission  of  the  radiant  heat  being  thus 
prevented.  The  cold  gas  is  intensely  opaque  to  the 
radiation  from  this  particular  flame,  though  highly 
transparent  to  heat  of  every  other  kind.  We  are  here 
manifestly  dealing  with  that  great  principle  which  lies 
at  the  basis  of  spectrum  analysis,  and  which  has  enabled 
scientific  men  to  determine  the  substances  of  which  the 
sun,  the  stars,  and  even  the  nebulae  are  composed ;  the 
principle,  namely,  that  a  body  which  is  competent  to 
emit  any  ray,  whether  of  heat  or  light,  is  competent  in  the 
same  degree  to  absorb  that  ray.  The  absorption  depends 
on  the  synchronism  existing  between  the  vibrations  of  the 
atoms  from  which  the  rays,  or  more  correctly  the  waves, 
issue,  and  those  of  the  atoms  on  which  they  impinge. 

To  its  almost  total  incompetence  to  emit  white  light, 
aqueous  vapour  adds  a  similar  incompetence  to  absorb 
white  light.  It  cannot,  for  example,  absorb  the  lumi- 
nous rays  of  the  sun,  though  it  can  absorb  the  non-lumi- 
nous rays  of  the  earth.  This  incompetence  of  the  vapour 
to  absorb  luminous  rays  is  shared  by  water  and  ice — in 
fact,  by  all  really  transparent  substances.  Tlieir  trans- 
parency is  due  to  their  inability  to  absorb  luminous  rays. 
The  molecules  of  such  substances  are  in  dissonance  with 
the  luminous  waves  ;  and  hence  such  waves  pass  through 
transparent  bodies  without  disturbing  the  molecular 
rest.     A  purely  luminous  beam,  however  intense  may 


84  FRAGMENTS  OF  SCIENCE. 

be  its  heat,  is  sensibly  incompetent  to  melt  ice.  "We 
can,  for  example,  converge  a  powerful  luminous  beam 
upon  a  surface  covered  with  hoar  frost,  without  melting 
a  single  spicula  of  the  crystals.  How  then,  it  may  be 
asked,  are  the  snows  of  the  Alps  swept  away  by  the  sun- 
shine of  summer  ?  I  answer,  they  are  not  swept  away 
by  sunshine  at  all,  but  by  rays  which  have  no  sunshine 
whatever  in  them.  The  luminous  rays  of  the  sun  fall 
upon  the  snow-fields  and  are  flashed  in  echoes  from 
crystal  to  crystal,  but  they  find  next  to  no  lodgment 
within  the  crystals.  They  are  hardly  at  all  absorbed,  and 
hence  they  cannot  produce  fusion.  But  a  body  of  power- 
ful dark  rays  is  emitted  \  y  the  sun  ;  and  it  is  these  that 
cause  the  glaciers  to  shrink  and  the  snows  to  disappear ; 
it  is  they  that  fill  the  banks  of  the  Arve  and  Arveyron, 
and  liberate  from  their  frozen  captivity  the  Ehone  and 
the  Rhine. 

Placing  a  concave  silvered  mirror  behind  the  electric 
light  its  rays  are  converged  to  a  focus  of  dazzling  bril- 
liancy. Placing  in  the  path  of  the  rays,  between  the 
light  and  the  focus,  a  vessel  of  water,  and  introducing 
at  the  focus  a  piece  of  ice,  the  ice  is  not  melted  by  the 
concentrated  beam.  Matches,  at  the  same  place,  are 
ignited,  and  wood  is  set  on  fire.  The  powerful  heat, 
then,  of  this  luminous  beam  is  incompetent  to  melt  the 
ice.  On  withdrawing  the  cell  of  water,  the  ice  imme- 
diately liquefies,  and  the  water  trickles  from  it  in  drops. 
Reintroducing  the  cell  of  water,  the  fusion  is  arrested, 
and  the  drops  cease  to  fall.  The  transparent  water  of 
the  cell  exerts  no  sensible  absorption  on  the  luminous 
rays,  still  it  withdraws  something  from  the  beam,  which, 
when  permitted  to  act,  is  competent  to  melt  the  ice. 
This  something  is  the  dark  radiation  of  the  electric 
light.  Again,  I  place  a  slab  of  pure  ice  in  front  of  the 
electric  lamp ;  send  a  luminous  beam  first  through  our 


RADIANT  HEAT  AND  ITS  RELATIONS.  85 

cell  of  water  and  then  through  the  ice.  By  means  of 
a  lens  an  image  of  the  slab  is  cast  upon  a  white  screen. 
The  beam,  sifted  by  the  water,  has  little  power  upon  the 
ice.  But  observe  what  occurs  when  the  water  is  re- 
moved ;  we  have  here  a  star  and  there  a  star,  each  star 
resembling  a  flower  of  six  petals,  and  growing  visibly 
laiger  before  our  eyes.  As  the  leaves  enlarge,  their 
edges  become  serrated,  but  there  is  no  deviation  from 
the  six-rayed  type.  We  have  here,  in  fact,  the  crystal- 
lisation of  the  ice  reversed  by  the  invisible  rays  of  the 
electric  beam.  They  take  the  molecules  down  in  this 
wonderful  way,  and  reveal  to  us  the  exquisite  atomic 
structure  of  the  substance  with  which  Nature  every 
winter  roofs  our  ponds  and  lakes. 

Numberless  effects,  apparently  anomalous,  might  be 
adduced  in  illustration  of  the  action  of  these  lightless 
rays.  These  two  powders,  for  example,  are  both  white, 
and  undistinguishable  from  each  other  by  the  eye.  The 
luminous  rays  of  the  sun  are  unabsorbed  by  both — from 
such  rays  these  powders  acquire  no  heat ;  still  one  of 
them,  sugar,  is  heated  so  highly  by  the  concentrated 
beam  of  the  electric  lamp,  that  it  first  smokes  and  then 
violently  inflames,  while  the  other  substance,  salt,  is 
barely  warmed  at  the  focus.  Placing  two  perfectly 
transparent  liquids  in  test-tubes  at  the  focus,  one  of 
them  boils  in  a  couple  of  seconds,  while  the  other,  in  a 
similar  position,  is  hardly  warmed.  The  boiling-point 
of  the  first  liquid  is  78°  C,  which  is  speedily  reached; 
that  of  the  second  liquid  is  only  48°  C,  which  is  never 
reached  at  all.  These  anomalies  are  entirely  due  to  the 
unseen  element  which  mingles  with  the  luminous  rays 
of  the  electric  beam,  and  indeed  constitutes  90  per  cent, 
of  its  calorific  power. 

A  substance,  as  many  of  you  know,  has  been  dis- 
covered, by  which  these  dark  rays  may  be  detached  from 


86  FRAGMENTS  OF  SCIENCE. 

the  total  emission  of  tlie  electric  lamp.  This  ray-filtei 
is  a  liquid,  black  as  pitch  to  the  luminous,  but  bright 
as  a  diamond  to  the  non-luminous,  radiation.  It  mer- 
cilessly cuts  off  the  former,  but  allows  the  latter  free 
transmission.  When  these  invisible  rays  are  brought 
to  a  focus,  at  a  distance  of  several  feet  from  the  electric 
lamp,  the  dark  rays  form  an  invisible  image  of  their 
source.  By  proper  means,  this  image  may  be  trans- 
formed into  a  visible  one  of  dazzling  brightness.  It 
might,  moreover,  be  shown,  if  time  permitted,  how,  out 
of  those  perfectly  dark  rays,  could  be  extracted,  by  a 
process  of  transmutation,  all  the  colours  of  the  solar 
spectrum.  It  might  also  be  proved  that  those  rays, 
powerful  as  they  are,  and  sufficient  to  fuse  many  metals, 
can  be  permitted  to  enter  the  eye,  and  to  break  upon 
the  retina,  without  producing  the  least  luminous  im- 
pression. 

The  dark  rays  being  thus  collected,  you  see  nothing 
at  their  place  of  convergence.  With  a  proper  thermo- 
meter it  could  be  proved  that  even  the  air  at  the  focus 
is  just  as  cold  as  the  surrounding  air.  And  mark  the 
conclusion  to  which  this  leads.  It  proves  the  ether  at 
the  focus  to  be  practically  detached  from  the  air, — that 
the  most  violent  ethereal  motion  may  there  exist, 
without  the  least  aerial  motion.  But,  though  you  see 
it  not,  there  is  sufficient  heat  at  that  focus  to  set 
London  on  fire.  The  heat  there  is  competent  to  raise 
iron  to  a  temperature  at  which  it  throws  off  brilliant 
scintillations.  It  can  heat  platinum  to  whiteness,  and 
almost  fuse  that  refractory  metal.  It  actually  can  fuse 
gold,  silver,  copper,  and  aluminium.  The  moment, 
moreover,  that  wood  is  placed  at  the  focus  it  bursts  into 
a  blaze. 

It  has  been  already  affirmed  that,  whether  as  re- 
gards radiation  or  absorption,  the   elementary  atoms 


RADIANT  HEAT  AND  ITS  RELATIONS.  87 

possess  but  little  power.  This  might  be  illustrated  by 
a  long  array  of  facts ;  and  one  of  the  most  'singular  of 
these  is  furnished  by  the  deportment  of  that  extremely 
combustible  substance,  phosphorus,  when  placed  at  the 
dark  focus.  It  is  impossible  to  ignite  there  a  fragment 
of  amorphous  phosphorus.  But  ordinary  phosphorus  is 
a  far  quicker  combustible,  and  its  deportment  towards 
radiant  heat  is  still  more  impressive.  It  may  be  ex- 
posed to  the  intense  radiation  of  an  ordinary  fire  with- 
out bursting  into  flame.  It  may  also  be  exposed  for 
twenty  or  thirty  seconds  at  an  obscure  focus,  of  suffi- 
cient power  to  raise  platinum  to  a  red  heat,  without 
ignition.  Notwithstanding  the  energy  of  the  ethereal 
waves  here  concentrated,  notwithstanding  the  extremely 
inflammable  character  of  the  elementary  body  exposed 
to  their  action,  the  atoms  of  that  body  refuse  to  partake 
of  the  motion  of  the  powerful  waves  of  low  refrangi- 
bility,  and  consequently  cannot  be  affected  by  their  heat. 
The  knowledge  we  now  possess  will  enable  us  to 
analyse  with  profit  a  practical  question.  White  dresses 
are  worn  in  summer,  because  they  are  found  to  be 
cooler  than  dark  ones.  The  celebrated  Benjamin 
Franklin  placed  bits  of  cloth  of  various  colours  upon 
snow,  exposed  them  to  direct  sunshine,  and  found  that 
they  sank  to  difi'erent  depths  in  the  snow.  The  black 
cloth  sank  deepest,  the  white  did  not  sink  at  all. 
Franklin  inferred  from  this  experiment  that  black 
bodies  are  the  best  absorbers,  and  white  ones  the  worst 
absorbers,  of  radiant  heat.  Let  us  test  the  generality 
of  this  conclusion.  One  of  these  two  cards  is  coated 
with  a  very  dark  powder,  and  the  other  with  a  perfectly 
white  one.  I  place  the  powdered  surfaces  before  a  fire, 
and  leave  them  there  until  they  have  acquired  as  high 
a  temperature  as  they  can  attain  in  this  position. 
Which  of  the  cards  is  then  most  highly  heated  ?     It 

r 


88  FRAGMENTS  OF  SCIENCE. 

requires  no  thermometer  to  answer  this  question. 
Simply  pressing  the  back  of  the  card,  on  which  the 
white  powder  is  strewn,  against  the  cheek  or  fore- 
head, it  is  found  intolerably  hot.  Placing  the  dark 
cani  in  the  same  position,  it  is  found  cool.  The 
white  powder  has  absorbed  far  more  heat  than  the 
dark  one.  This  simple  result  abolishes  a  hundred 
conclusions  which  have  been  hastily  drawn  from 
the  experiment  of  Franklin.  Again,  liere  are  sus- 
pended two  delicate  mercurial  thermometers  at  the 
same  distance  from  a  gas- flame.  The  bulb  of  one  of 
them  is  covered  by  a  dark  substance,,  the  bulb  of  the 
other  by  a  white  one.  Both  bulbs  have  received  the 
radiation  from  the  flame,  but  the  white  bulb  has 
absorbed  most,  and  its  mercury  stands  much  higher 
than  that  of  the  other  thermometer.  This  experiment 
might  be  varied  in  a  hundred  ways :  it  proves  that 
from  the  darkness  of  a  body  you  can  draw  no  certain 
conclusion  regarding  its  power  of  absorption. 

The  reason  of  this  simply  is,  that  colour  gives  us 
intelligence  of  only  one  portion,  and  that  the  smallest 
one,  of  the  rays  impinging  on  the  coloured  body. 
Were  the  rays  all  luminous,  we  might  with  certainty 
infer  from  the  colour  of  a  body  its  power  of  absorption ; 
but  the  great  mass  of  the  radiation  from  our  fire,  our 
gas-flame,  and  even  from  the  sun  itself,  consists  of 
invisible  calorific  rays,  regarding  which  colour  teaches 
us  nothing.  A  body  may  be  highly  transparent  to  the 
one  class  of  rays,  and  highly  opaque  to  the  other.  Thus 
the  white  powder,  which  has  shown  itself  so  powerful 
an  absorber,  has  been  specially  selected  on  account  of 
its  extreme  perviousness  to  the  visible  rays,  and  its 
extreme  imperviousness  to  the  invisible  ones;  while 
the  dark  powder  was  chosen  on  accoimt  of  its  extreme 
transparency  to  the  invisible,  and  its  extreme  opacity 


RADIANT  HEAT  AND  ITS  RELATIONS.  89 

to  the  visible,  rays.     In  the  case  of  the  radiation  from 
our  fire,  about  98  per  cent,  of  the  whole  emission  con 
sists  of  invisible  rays ;  the  body,  therefore,  which  was 
most  opaque  to  these  triumphed  as  an  absorber,  though 
that  body  was  a  white  one. 

And  here  it  is  worth  while  to  consider  the  manner 
in  which  we  obtain  from  natural  facts  what  may  be 
called  their  intellectual  value.  Throughout  the  pro- 
cesses of  Nature  we  have  interdependence  and  harmony ; 
and  the  main  value  of  physics,  considered  as  a  mental 
discipline,  consists  in  the  tracing  out  of  this  inter- 
dependence, and  the  demonstration  of  this  harmony. 
The  outward  and  visible  phenomena  are  the  counters 
of  the  intellect ;  and  our  science  would  not  be  worthy 
of  its  name  and  fame  if  it  halted  at  facts,  however 
practically  useful,  and  neglected  the  laws  which  accom- 
pany and  rule  the  phenomena.  Let  us  endeavour, 
then,  to  extract  from  the  experiment  of  Franklin  all 
that  it  can  yield,  calling  to  our  aid  the  knowledge 
which  our  predecessors  have  already  stored.  Let  us 
imagine  two  pieces  of  cloth  of  the  same  texture,  the 
one  black  and  the  other  white,  placed  upon  sunned 
snow.  Fixing  our  attention  on  the  white  piece,  let  us 
enquire  whether  there  is  any  reason  to  expect  that  it 
will  sink  in  the  snow  at  all.  There  is  knowledge  at 
hand  which  enables  us  to  reply  at  once  in  the  negative. 
There  is,  on  the  contrary,  reason  to  expect  that,  after  a 
sufficient  exposure,  the  bit  of  cloth  will  be  found  on  an 
eminence  instead  of  in  a  hollow ;  that  instead  of  a 
depression,  we  shall  have  a  relative  elevation  of  the 
bit  of  cloth.  For,  as  regards  the  luminous  rays  of  the 
sun,  the  cloth  and  the  snow  are  alike  powerless ;  the 
one  cannot  be  warmed,  nor  the  other  melted,  by  such 
rays.  The  cloth  is  white  and  the  snow  is  white,  because 
their  confusedly  mingled  fibres  and  particles  are  incom- 


90  FRAGMENTS  OF  SCIENCE.* 

petent  to  absorb  the  luminous  rays.  Whether,  then, 
the  cloth  will  sink  or  not  depends  entirely  upon  the 
dark  rays  of  the  sun.  Now  the  substance  which  absorbs 
these  dark  rays  with  the  greatest  avidity  is  ice, — or 
snow,  which  is  merely  ice  in  powder.  Hence,  a  less 
amount  of  heat  will  be  lodged  in  the  cloth  than  in  the 
surrounding  snow.  The  cloth  must  therefore  act  as  a 
shield  to  the  snow  on  which  it  rests ;  and,  in  consequence 
of  the  more  rapid  fusion  of  the  exposed  snow,  its  shield 
must,  in  due  time,  be  left  behind,  perched  upon  an 
eminence  like  a  glacier-table. 

But  though  the  snow  transcends  the  cloth,  both  as  a 
radiator  and  absorber,  it  does  not  much  transcend  it. 
Cloth  is  very  powerful  in  both  these  respects.  Let  us 
now  turn  our  attention  to  the  piece  of  black  cloth,  the 
texture  and  fabric  of  which  I  assume  to  be  the  same  as 
that  of  the  white.  For  our  object  being  to  compare 
the  efifects  of  colour,  we  must,  in  order  to  study  this 
effect  in  its  purity,  preserve  all  the  other  conditions 
constant.  Let  us  then  suppose  the  black  cloth  to  be 
obtained  from  the  dyeing  of  the  white.  The  cloth 
itself,  without  reference  to  the  dye,  is  nearly  as  good  an 
absorber  of  heat  as  the  snow  around  it.  But  to  the 
absorption  of  the  dark  solar  rays  by  the  undyed  cloth,  is 
now  added  the  absorption  of  the  whole  of  the  luminous 
rays,  and  this  great  additional  influx  of  heat  is  far  more 
than  sufficient  to  turn  the  balance  in  favour  of  the 
black  cloth.  The  sum  of  its  actions  on  the  dark  and 
luminous  rays,  exceeds  the  action  of  the  snow  on  the 
dark  rays  alone.  Hence  the  cloth  will  sink  in  the 
snow,  and  this  is  the  complete  analysis  of  Franklin's 
experiment. 

Throughout  this  discourse  the  main  stress  has  been 
laid  on  chemical  constitution,  as  influencing  most 
powerfully  the  phenomena  of  radiation  and  absorption- 


RADIANT  HEAT  AND  ITS  RELATIONS.  91 

With  regard  to  gases  and  vapours,  and  to  the  liquids 
from  which  these  vapours  are  derived,  it  has  been 
proved  by  the  most  varied  and  conclusive  experiments 
that  the  acts  of  radiation  and  absorption  are  molecula/r 
— that  they  depend  upon  chemical,  and  not  upon 
mechanical,  condition.  In  attempting  to  extend  this 
principle  to  solids  I  was  met  by  a  multitude  of  facts, 
obtained  by  celebrated  experimenters,  which  seemed 
flatly  to  forbid  such  an  extension.  Melloni,  for  example, 
had  found  the  same  radiant  and  absorbent  power  for 
chalk  and  lamp-black.  MM.  Masson  and  Courtepee 
had  performed  a  most  elaborate  series  of  experiments  on 
chemical  precipitates  of  various  kinds,  and  found  that 
they  one  and  all  manifested  the  same  power  of  radiation. 
They  concluded  from  their  researches,  that  when  bodies 
are  reduced  to  an  extremely  fine  state  of  division,  the 
influence  of  this  state  is  so  powerful  as  entirely  to  mask 
and  override  whatever  influence  may  be  due  to  chemical 
constitution. 

But  it  appears  to  me  that  through  the  whole  of  these 
researches  an  oversight  has  run,  the  mere  mention  of 
which  will  show  what  caution  is  essential  in  the  opera- 
tions of  experimental  philosophy ;  while  an  experiment 
or  two  will  make  clear  wherein  the  oversight  consists. 
Filling  a  brightly  polished  metal  cube  with  boiling 
water,  I  determine  the  quantity  of  heat  emitted  by  two 
of  the  bright  surfaces.  As  a  radiator  of  heat  one  of 
them  far  transcends  the  other.  Both  surfaces  appear  to 
be  metallic ;  what,  then,  is  the  cause  of  the  observed 
difference  in  their  radiative  power  ?  Simply  this :  one 
of  the  surfaces  is  coated  with  transparent  gum,  through 
which,  of  course,  is  seen  the  metallic  lustre  behind ;  and 
this  varnish,  though  so  perfectly  transparent  to  luminous 
rays,  is  as  opaque  as  pitch,  or  lamp-black,  to  non-lumi- 
nous ones.     It  is  a  powerfuj  emitter  of  dark  rays ;  it 


92  FRAGMENTS  OF  SCIEKCE. 

is  also  a  powerful  absorber.  While,  therefore,  at  the 
present  moment,  it  is  copiously  pouring  forth  radiant 
heat  itself,  it  does  not  allow  a  single  ray  from  the  metal 
behind  to  pass  through  it.  The  varnish  then,  and  not 
the  metal,  is  the  real  radiator. 

Now  Melloni,  and  Masson,  and  Courtepee  experi- 
mented thus :  they  mixed  their  powders  and  precipi- 
tates with  gum-water,  and  laid  them,  by  means  of  a 
brush,  upon  the  surfaces  of  a  cube  like  this.  True, 
they  saw  their  red  powders  red,  their  white  ones  white, 
and  their  black  ones  black,  but  they  saw  these  colours 
through  the  coat  of  varnish  which  surrounded  every 
'pa/rticle.  When,  therefore,  it  was  concluded  that 
colour  had  no  influence  on  radiation,  no  chance  had 
been  given  to  it  of  asserting  its  influence ;  when  it 
was  found  that  all  chemical  precipitates  radiated 
alike,  it  was  the  radiation  from  a  varnish,  common 
to  them  all,  which  showed  the  observed  constancy. 
Hundreds,  perhaps  thousands,  of  experiments  on 
radiant  heat  have  been  performed  in  this  way,  by 
various  enquirers,  but  the  work  will,  I  fear,  have  to  be 
done  over  again.  I  am  not,  indeed,  acquainted  with  an 
instance  in  which  an  oversight  of  so  trivial  a  character 
has  been  committed  by  so  many  able  men  in  succession, 
vitiating  so  large  an  amount  of  otherwise  excellent  work. 

Basing  our  reasonings  thus  on  demonstrated  facts, 
we  arrive  at  the  extremely  probable  conclusion  that 
the  envelope  of  the  particles,  and  not  the  particles 
themselves,  was  the  real  radiator  in  the  experiments 
just  referred  to.  To  reason  thus,  and  deduce  their 
more  or  less  probable  consequences  from  experimental 
facts,  is  an  incessant  exercise  of  the  student  of  physical 
science.  But  having  thus  followed,  for  a  time,  the 
light  of  reason  alone  through  a  series  of  phenomena, 
and   emerged   from   them^  with  a  piurely   intellectual 


RADIANT  HEAT  AND  ITS  EELATIONS.  93 

conclusion,  our  duty  is  to  bring  that  conclusion  to  an 
experimental  test.  In  this  way  we  fortify  our  science. 
For  the  purpose  of  testing  our  conclusion  regarding 
the  influence  of  the  gum,  I  take  two  powders  presenting 
the  same  physical  appearance ;  one  of  them  is  a  com- 
pound of  mercury,  and  the  other  a  compound  of  lead. 
On  two  surfaces  of  a  cube  are  spread  these  bright  red 
powders,  without  varnish  of  any  kind.  Filling  the 
cube  with  boiKng  water,  and  determining  the  radiation 
from  the  two  surfaces,  one  of  them  is  found  to  emit 
thirty-nine  units  of  heat,  while  the  other  emits  seventy- 
four.  This,  surely,  is  a  great  difference.  Here,  how- 
ever, is  a  second  cube,  having  two  of  its  surfaces  coated 
with  the  same  powders,  the  only  difference  being  that 
the  powders  are  laid  on  by  means  of  a  transparent  gum. 
Both  surfaces  are  now  absolutely  alike  in  radiative 
power.  Both  of  them  emit  somewhat  more  than  was 
emitted  by  either  of  the  unvarnished  powders,  simply 
because  the  gum  employed  is  a  better  radiator  than 
either  of  them.  Excluding  all  varnish,  and  comparing 
white  with  white,  vast  differences  are  found  ;  comparing 
black  with  black,  they  are  also  different;  and  when 
black  and  white  are  compared,  in  some  cases  the  black 
radiates  far  more  than  the  white,  while  in  other  cases 
the  white  radiates  far  more  than  the  black.  Deter- 
mining, moreover,  the  absorptive  power  of  those 
powders,  it  is  found  to  go  hand-in-hand  with  their 
radiative  power.  The  good  radiator  is  a  good  absorber, 
and  the  bad  radiator  is  a  bad  absorber.  From  all  this 
it  is  evident  that  as  regards  the  radiation  and  absorp- 
tion of  non-luminous  heat,  colour  teaches  us  nothing ; 
and  that  even  as  regards  the  radiation  of  the  sun, 
consisting  as  it  does  mainly  of  non-luminous  rays, 
conclusions  as  to  the  influence  of  colour  may  be  alto- 
gether delusive.     This  is  the  strict  scientific  upshot  of 


94  FRAGMENTS  OF  SCIENCE. 

our  researches.  But  it  is  not  the  less  true  that  in  the 
case  of  wearing  apparel — and  this  for  reasons  which  I 
have  given  in  analysing  the  experiment  of  Franklin- - 
black  dresses  are  more  potent  than  white  ones  as  ab- 
sorbers of  solar  heat. 

Thus,  in  biief  outline,  have  been  brought  before 
you  a  few  of  the  results  of  recent  enquiry.  If  you  ask 
me  what  is  the  use  of  them,  I  can  hardly  answer  you, 
unless  you  define  the  term  use.  If  you  meant  to  ask 
whether  those  dark  rays  which  clear  away  the  Alpine 
snows,  will  ever  be  applied  to  the  roasting  of  turkeys, 
or  the  driving  of  steam-engines — while  affirming  their 
power  to  do  both,  I  would  frankly  confess  that  tliey 
are  not  at  present  capable  of  competing  profitably  with 
coal  in  these  particulars.  Still  they  may  have  great 
uses  unknown  to  me;  and  when  our  coal-fields  are 
exhausted,  it  is  possible  that  a  more  ethereal  race 
than  we  are  may  cook  their  victuals,  and  perform  their 
work,  in  this  transcendental  way.  But  is  it  necessary 
that  the  student  of  science  should  have  his  labours 
tested  by  their  possible  practical  applications  ?  What 
is  the  practical  value  of  Homer's  Iliad  ?  You  smile, 
and  possibly  think  that  Homer's  Iliad  is  good  as  a 
means  of  culture.  There's  the  rub.  The  people  who 
demand  of  science  practical  uses,  forget,  or  do  not 
know,  that  it  also  is  great  as  a  means  of  culture — that 
the  knowledge  of  this  wonderful  universe  is  a  thing 
profitable  in  itself,  and  requiring  no  practical  applica- 
tion to  justify  its  pursuit. 

But  while  the  student  of  Nature  distinctly  refuses 
to  have  his  labours  judged  by  their  practical  issues, 
unless  the  term  practical  be  made  to  include  mental 
as  well  as  material  good,  he  knows  full  well  that  the 
greatest  practical  triumphs  have  been  episodes  in  the 
search  after  pure  natural  truth.     The  electric  telegraph 


RADIANT  HEAT  AND  ITS  RELATIONS.  95 

is  the  standing  wonder  of  this  age,  and  the  men  whose 
scientific  knowledge,  and  mechanical  skill,  have  made 
the  telegraph  what  it  is,  are  deserving  of  all  honour. 
In  fact,  they  have  had  their  reward,  both  in  reputation 
and  in  those  more  substantial  benefits  which  the  direct 
service  of  the  public  always  carries  in  its  train.  But 
whoj  I  would  ask,  put  the  soul  into  this  telegraphic 
body  ?  Who  snatched  from  heaven  the  fire  that  flashes 
along  the  line  ?  This,  I  am  bound  to  say,  was  done 
by  two  men,  the  one  a  dweller  in  Italy,*  the  other  a 
dweller  in  England,^  who  never  in  their  enquiries 
consciously  set  a  practical  object  before  them — whose 
only  stimulus  was  the  fascination  which  draws  the 
climber  to  a  never-trodden  peak,  and  would  have  made 
Caesar  quit  his  victories  for  the  sources  of  the  Nile. 
That  the  knowledge  brought  to  us  by  those  prophets, 
priests,  and  kings  of  science  is  what  the  world  calls 
'  useful  knowledge,'  the  triumphant  application  of  their 
discoveries  proves.  But  science  has  another  function 
to  fulfil,  in  the  storing  and  the  training  of  the  human 
mind;  and  I  would  base  my  appeal  to  you  on  the 
specimen  which  has  this  evening  been  brought  before 
you,  whether  any  system  of  education  at  the  present 
day  can  be  deemed  even  approximately  complete,  in 
which  the  knowledge  of  Nature  is  neglected  or  ignored. 
^  Volta.  ^  Faraday. 


IV. 

NEW  CHEMICAL  REACTIONS  PRODUCED  BY 
LIGHT. 

1868-69. 

Measured  by  their  power,  not  to  excite  vision,  but  to 
produce  heat — in  other  words,  measured  by  their  ab- 
solute energy — the  ultra-red  waves  of  the  sun  and  of 
the  electric  light,  as  shown  in  the  preceding  articles, 
far  transcend  the  visible.  In  the  domain  of  chemistry, 
however,  there  are  numerous  cases  in  which  the  more 
powerful  waves  are  ineffectual,  while  the  more  minute 
waves,  through  what  may  be  called  their  timeliness  of 
application,  are  able  to  produce  great  efifects.  A  series 
of  these,  of  a  novel  and  beautiful  character,  discovered 
in  1868,  and  further  illustrated  in  subsequent  years, 
may  be  exhibited  by  subjecting  the  vapours  of  volatile 
liquids  to  the  action  of  concentrated  sunlight,  or  to 
the  concentrated  beam  of  the  electric  light.  Their 
investigation  led  up  to  the  discourse  on  '  Dust  and 
Disease'  which  follows  in  this  volume;  and  for  this 
reason  some  account  of  them  is  introduced  here. 

A  glass  tube  3  feet  long  and  3  inches  wide,  which 
had  been  frequently  employed  in  my  researches  on 
radiant  heat,  was  supported  horizontally  on  two  stands. 
At  one  end  of  the  tube  was  placed  an  electric  lamp, 
the  height  and  position  of  both  being  so  arranged,  that 
the  axis  of  the  tube,  and  that  of  the  beam  issuing  from 


DECOMPOSITION  BY  LIGHT. 


97 


Pia.  2. 


the  lamp,  were  coincident.  In  the  first  experiments 
the  two  ends  of  the  tube  were  closed  by  plates  of  rock- 
salt,  and  subsequently  by  plates  of  glass.  For  the  sake 
of  distinction,  I  call  this  tube  the  experimental  tube. 
It  was  connected  with  an  air-pump,  and  also  with  a 
series  of  drying  and  other  tubes  used  for  the  purifica- 
tion of  the  air. 

A  number  of  test-tubes,  like  F,  fig.  2  (I  have  used 
at  least  fifty  of  them),  were  converted  into  Woulf's 
flasks.  Each  of  them  was  stopped 
by  a  cork,  through  which  passed  ^j 
two  glass  tubes:  one  of  these 
tubes  (a)  ended  immediately 
below  the  cork,  while  the  other 
(b)  descended  to  the  bottom  of 
the  flask,  being  drawn  out  at  its 
lower  end  to  an  orifice  about 
0*03  of  an  inch  in  diameter.  It 
was  found  necessary  to  coat  the 
cork  carefully  with  cement.  In 
the  later  experiments  corks  of 
vulcanised  india-rubber  were  in- 
variably employed. 

The  little  flask,  thus  formed, 
being  partially  filled  with  the 
liquid  whose  vapour  was  to  be" 
examined,  was  introduced  into 
the  path  of  the  purified  current 
of  air.  The  experimental  tube 
being  exhausted,  and  the  cock 
which  cut  off  the  supply  of 
purified  air  being  cautiously 
turned  on,  the  air  entered  the  flask  through  the  tube  b, 
and  escaped  by  the  small  orifice  at  the  lower  end  of 
b  into  the  liquid.     Through  this  it  bubbled,  loading 


FRAGMENTS  OF  SCIENCE. 


DECOMPOSITION  BY  LIGHT.  99 

itself  with  vapour,  after  which  the  mixed  air  and 
vapour,  passing  from  the  flask  by  the  tube  a,  entered 
the  experimental  tube,  where  they  were  subjected  to 
the  action  of  light. 

The  whole  arrangement  is  shown  in  fig.  3,  where  L 
represents  the  electric  lamp,  s  s'  the  experimental  tube, 
pp'  the  pipe  leading  to  the  air-pump,  and  F  the  test- 
tube  containing  the  volatile  liquid.  The  tube  t  f  is 
plugged  with  cotton-wool  intended  to  intercept  the  float- 
ing matter  of  the  air ;  the  bent  tube  t'  contains  caustic 
potash,  the  tube  t  sulphuric  acid,  the  one  intended  to 
remove  the  carbonic  acid  and  the  other  the  aqueous 
vapour  of  the  air. 

The  power  of  the  electric  beam  to  reveal  the 
existence  of  anything  within  the  experimental  tube,  or 
the  impurities  of  the  tube  itself,  is  extraordinary. 
When  the  experiment  is  made  in  a  darkened  room,  a 
tube  which  in  ordinary  daylight  appears  absolutely 
clean,  is  often  shown  by  the  present  mode  of  examina- 
tion to  be  exceedingly  filthy. 

The  following  are  some  of  the  results  obtained  with 
this  arrangement : — 

Nitrite  of  amyl. — The  vapour  of  this  liquid  was  in 
the  first  instance  permitted  to  enter  the  experimental 
tube,  while  the  beam  from  the  electric  lamp  was 
passing  through  it.  Curious  clouds,  the  cause  of  which 
was  then  unknown,  were  observed  to  form  near  the 
place  of  entry,  being  afterwards  whirled  through  the 
tube. 

The  tube  being  again  exhausted,  the  mixed  air  and 
vapour  were  allowed  to  enter  it  in  the  dark.  The 
slightly  convergent  beam  of  the  electric  light  was  then 
sent  through  the  mixture.  For  a  moment  the  tube 
was  optically  empty,  nothing  whatever  being  seen 
within  it ;  but  before  a  second  had  elapsed  a  shower  of 


100  FRAGMENTS  OF  SCIENCE. 

particles  was  precipitated  on  the  beam.  The  cloud 
thus  generated  became  denser  as  the  light  continued 
to  act,  showing  at  some  places  vivid  iridescence. 

The  lens  of  the  electric  lamp  was  now  placed  so  as 
to  form  within  the  tube  a  strongly  convergent  cone  of 
rays.  The  tube  was  cleansed  and  again  filled  in  dark- 
ness. When  the  light  was  sent  through  it,  the  pre- 
cipitation upon  the  beam  was  so  rapid  and  intense  that 
the  cone,  which  a  moment  before  was  invisible,  flashed 
suddenly  forth  like  a  solid  luminous  spear.  The  effect 
was  the  same  when  the  air  and  vapour  were  allowed  to 
enter  the  tube  in  diffuse  daylight.  The  cloud,  however, 
which  shone  with  such  extraordinary  radiance  under 
the  electric  beam,  was  invisible  in  the  ordinary  light  of 
the  laborat.ory. 

The  quantity  of  mixed  air  and  vapour  within  the 
experimental  tube  could  of  course  be  regulated  at 
pleasure.  The  rapidity  of  the  action  diminished  with 
the  attenuation  of  the  vapour.  When,  for  example, 
the  mercurial  column  associated  with  the  experimental 
tube  was  depressed  only  five  inches,  the  action  was  not 
nearly  so  rapid  as  when  the  tube  was  full.  In  such 
cases,  however,  it  was  exceedingly  interesting  to  ob- 
serve, after  some  seconds  of  waiting,  a  thin  streamer  of 
delicate  bluish-white  cloud  slowly  forming  along  the 
axis  of  the  tube,  and  finally  swelling  so  as  to  fill  it. 

When  dry  oxygen  was  employed  to  carry  in  the 
vapour,  the  effect  was  the  same  as  that  obtained  with 
air. 

When  dry  hydrogen  was  used  as  a  vehicle,  the 
effect  was  also  the  same. 

The  effect,  therefore,  is  not  due  to  any  interaction 
between  the  vapour  of  the  nitrite  and  its  vehicle. 

This  was  further  demonstrated  by  the  deportment 
of  the  vapour  itself.     When  it  was  permitted  to  enter 


DECOMPOSITION  BY  LIGHT,  101 

the  experimental  tube  unmixed  with  air  or  any  other 
gas,  the  effect  was  substantially  the  same.  Hence  the 
seat  of  the  observed  action  is  the  vapour. 

This  action  is  not  to  be  ascribed  to  heat.  As 
regards  the  glass  of  the  experimental  tube,  and  the  air 
within  the  tube,  the  beam  employed  in  these  experi- 
ments was  perfectly  cold.  It  had  been  sifted  by  passing 
it  through  a  solution  of  alum,  and  through  the  thick 
double-convex  lens  of  the  lamp.  When  the  unsifted 
beam  of  the  lamp  was  employed,  the  effect  was  still 
the  same  ;  the  obscure  calorific  rays  did  not  appear  to 
interfere  with  the  result. 

My  object  here  being  simply  to  point  out  to 
chemists  a  method  of  experiment  which  reveals  a  new 
and  beautiful  series  of  reactions,  I  left  to  them  the 
examination  of  the  products  of  decomposition.  The 
group  of  atoms  forming  the  molecule  of  nitrite  of  amyl 
is  obviously  shaken  asunder  by  certain  specific  waves  of 
the  electric  beam,  nitric  oxide  and  other  products,  of 
which  the  nitrate  of  amyl  is  probably  one,  being  the 
result  of  the  decomposition.  The  brown  fumes  of 
nitrous  acid  were  seen  mingling  with  the  cloud  within 
the  experimental  tube.  The  nitrate  of  amyl,  being 
less  volatile  than  the  nitrite,  and  not  being  able  to 
maintain  itself  in  the  condition  of  vapour,  would  be 
precipitated  as  a  visible  cloud  along  the  track  of  the 
beam. 

In  the  anterior  portions  of  the  tube  a  powerful  sift- 
ing of  the  beam  by  the  vapour  occurs,  which  diminishes 
the  chemical  action  in  the  posterior  portions.  In  some 
experiments  the  precipitated  cloud  only  extended  half- 
way down  the  tube.  When,  under  these  circumstances, 
the  lamp  was  shifted  so  as  to  send  the  beam  through 
the  other  end  of  the  tube,  copious  precipitation  occurred 
there  also. 


102  FRAGMENTS  OF  SCIENCE. 

Solar  light  also  effects  the  decomposition  of  the 
nitrite-of-amyl  vapour.  On  October  10,  1868,  I  par- 
tially darkened  a  small  room  in  the  Koyal  Institution, 
into  which  the  sun  shone,  permitting  the  light  to  enter 
through  an  open  portion  of  the  window-shutter.  In  the 
track  of  the  beam  was  placed  a  large  plano-convex  lens, 
which  formed  a  fine  convergent  cone  in  the  dust  of  the 
room  behind  it.  The  experimental  tube  was  filled  in 
the  laboratory,  covered  with  a  black  cloth,  and  carried 
into  the  partially  darkened  room.  On  thrusting  one  end 
of  the  tube  into  the  cone  of  rays  behind  the  lens,  pre- 
cipitation within  the  cone  was  copious  and  immediate. 
The  vapour  at  the  distant  end  of  the  tube  was  in  part 
shielded  by  that  in  front,  and  was  also  more  feebly 
acted  on  through  the  divergence  of  the  rays.  On  re- 
versing the  tube,  a  second  and  similar  cone  was  precipi- 
tated. 

Physical  ConsidercUiona, 

I  sought  to  determine  the  particular  portion  of  the 
light  which  produced  the  foregoing  effects.  When, 
previous  to  entering  the  experimental  tube,  the  beam 
was  caused  to  pass  through  a  red  glass,  the  effect  was 
greatly  weakened,  but  not  extinguished.  This  was  also 
the  case  with  various  samples  of  yellow  glass.  A  blue 
glass  being  introduced  before  the  removal  of  the  yellow 
or  the  red,  on  taking  the  latter  away  prompt  precipita- 
tion occurred  along  the  track  of  the  blue  beam.  Hence, 
in  this  case,  the  more  refrangible  rays  are  the  most 
chemically  active.  The  colour  of  the  liquid  nitrite  of 
amyl  indicates  that  this  must  be  the  case  ;  it  is  a  feeble 
but  distinct  yellow  :  in  other  words,  the  yellow  portion 
of  the  beam  is  most  freely  transmitted.  It  is  not, 
however,  the  transmitted  portion  of  any  beam  which 


DECOMPOSITION  BY  LIGHT.  103 

produces  chemical  action,  but  the  absorbed  portion. 
Blue,  as  the  complementary  colour  to  yellow,  is  here 
absorbed,  and  hence  the  more  energetic  action  of  the 
blue  rays. 

This  reasoning,  however,  assumes  that  the  same  rays 
are  absorbed  by  the  liquid  and  its  vapour.  The 
assumption  is  worth  testing.  A  solution  of  the  yellow 
chromate  of  potash,  the  colour  of  which  may  be  made 
almost,  if  not  altogether,  identical  with  that  of  the 
liquid  nitrite  of  amyl,  was  found  far  more  effective  in 
stopping  the  chemical  rays  than  either  the  red  or  the 
yellow  glass.  But  of  all  substances  the  liquid  nitrite 
itself  is  most  potent  in  arresting  the  rays  which  act 
upon  its  vapour.  A  layer  one-eighth  of  an  inch  in  thick- 
ness, which  scarcely  perceptibly  affected  the  luminous 
intensity,  absorbed  the  entire  chemical  energy  of  the 
concentrated  beam  of  the  electric  light. 

The  close  relation  subsisting  between  a  liquid  and 
its  vapour,  as  regards  their  action  upon  radiant  heat, 
has  been  already  amply  demonstrated.*  As  regards  the 
nitrite  of  amyl,  this  relation  is  more  specific  than  in  the 
cases  hitherto  adduced  ;  for  here  the  special  constituent 
of  the  beam,  which  provokes  the  decomposition  of  the 
vapour,  is  shown  to  be  arrested  by  the  liquid. 

A  question  of  extreme  importance  in  molecular 
physics  here  arises :  What  is  the  real  mechanism  of  this 
absorption,  and  where  is  its  seat  ?  ^  I  figure,  as  others 
do,  a  molecule  as  a  group  of  atoms,  held  together  by 
their  mutual  forces,  but  still  capable  of  motion  among 
themselves.     The  vapour  of  the  nitrite  of  amyl  is  to 

»  *  Phil.  Trans.'  1864  ;  •  Heat,  a  Mode  of  Motion,'  chap.  xii. ;  and 
p.  61  of  this  volume. 

^  My  attention  was  very  forcibly  directed  to  this  subject  some 
years  ago  by  a  conversation  with  my  excellent  friend  Professox 
Clnnsins. 

8 


104  FRAGMENTS  OF  SCIENCE. 

be  regarded  as  an  assemblage  of  such  molecules.  The 
question  now  before  us  is  this  :  In  the  act  of  absorption, 
is  it  the  molecules  that  are  effective,  or  is  it  their  con- 
stituent atoms?  Is  the  vis  viva  of  the  intercepted 
light-waves  transferred  to  the  molecule  as  a  whole,  or  to 
its  constituent  parts  ? 

The  molecule,  as  a  whole,  can  only  vibrate  in  virtue 
of  the  forces  exerted  between  it  and  its  neighbom:  mole- 
cules. The  intensity  of  these  forces,  and  consequently 
the  rate  of  vibration,  would,  in  this  case,  be  a  function 
of  the  distance  between  the  molecules.  Now  the  iden- 
tical absorption  of  the  liquid  and  of  the  vaporous  nitrite 
of  amyl  indicates  an  identical  vibrating  period  on  the 
part  of  liquid  and  vapour,  and  this,  to  my  mind, 
amounts  to  an  experimental  proof  that  the  absorption 
occurs  in  the  main  within  the  molecule.  For  it  can 
hardly  be  supposed,  if  the  absorption  were  the  act  of 
the  molecule  as  a  whole,  that  it  could  continue  to  aflect 
waves  of  the  same  period  after  the  substance  had  passed 
from  the  vaporous  to  the  liquid  state. 

In  point  of  fact,  the  decomposition  of  the  nitrite  of 
amyl  is  itself  to  some  extent  an  illustration  of  this  in- 
ternal molecular  absorption ;  for  were  the  absorption 
the  act  of  the  molecule  as  a  whole,  the  relative  motions 
of  its  constituent  atoms  would  remain  unchanged,  and 
there  would  be  no  mechanical  cause  for  their  separation. 
It  is  probably  the  synchronism  of  the  vibrations  of  one 
portion  of  the  molecule  with  the  incident  waves,  that 
enables  the  amplitude  of  those  vibrations  to  augment, 
until  the  chain  which  binds  the  parts  of  the  molecule 
together  is  snapped  asunder. 

I  anticipate  wide,  if  not  entire,  generality  for  the 
fact  that  a  liquid  and  its  vapour  absorb  the  same  rays. 
A  cell  of  liquid  chlorine  would,  I  imagine,  deprive  light 
more  effectually  of  its  power  of  causing  chlorine  and 


DECOMPOSITION  BY  LIGHT.  105 

hydrogen  to  combine  than  any  other  filter  of  the 
luminous  rays.  The  rays  which  give  chlorine  its  colour 
have  nothing  to  do  with  this  combination,  those  that 
are  absorbed  by  the  chlorine  being  the  really  effective 
rays.  A  highly  sensitive  bulb,  containing  chlorine  and 
hydrogen,  in  the  exact  proportions  necessary  for  the 
formation  of  hydrochloric  acid,  was  placed  at  one  end  of 
an  experimental  tube,  the  beam  of  the  electric  lamp 
being  sent  through  it  from  the  other.  The  bulb  did 
not  explode  when  the  tube  was  filled  with  chlorine, 
while  the  explosion  was  violent  and  immediate  when 
the  tube  was  filled  with  air.  I  anticipate  for  the  liquid 
chlorine  an  action  similar  to,  but  still  more  energetic 
than,  that  exhibited  by  the  gas.  If  this  should  prove 
to  be  the  case,  it  will  favour  the  view  that  chlorine 
itself  is  molecula/r  and  not  monatomic. 


Production   of   Sky-hlue   by   the  Decomposition  of 
Nitrite  of  AmyL 

When  the  quantity  of  nitrite  vapour  is  considerable, 
and  the  light  intense,  the  chemical  action  is  exceedingly 
rapid,  the  particles  precipitated  being  so  large  as  to 
whiten  the  luminous  beam.  Not  so,  however,  when  a 
well-mixed  and  highly  attenuated  vapour  fills  the  ex- 
perimental tube.  The  effect  now  to  be  described  was 
first  obtained  when  the  vapour  of  the  nitrite  was 
derived  from  a  portion  of  its  liquid  which  had  been 
accidentally  introduced  into  the  passage  through  which 
the  dry  air  flowed  into  the  experimental  tube. 

In  this  case,  the  electric  beam  traversed  the  tube  for 
several  seconds  before  any  action  was  visible.  Decom- 
position then  visibly  commenced,  and  advanced  slowly. 
When  the  light  was  very  strong,  the  cloud  appeared  of 


106  FRAGMENTS  OF  SCIENCE. 

a  milky  blue.  Wheu,  on  the  contrary,  the  intensity 
was  moderate,  the  blue  was  pure  and  deep.  In  Briicke's 
important  experiments  on  the  blue  of  the  sky  and  the 
morning  and  evening  red,  pure  mastic  is  dissolved  in 
alcohol,  and  then  dropped  into  water  well  stirred. 
When  the  proportion  of  mastic  to  alcohol  is  correct,  the 
resin  is  precipitated  so  finely  as  to  elude  the  highest 
microscopic  power.  By  reflected  light,  such  a  medium 
appears  bluish,  by  transmitted  light  yellowish,  which 
latter  colour,  by  augmenting  the  quantity  of  the  pre- 
cipitate, can  be  caused  to  pass  into  orange  or  red. 

But  the  development  of  colour  in  the  attenuated 
nitrite-of-amyl  vapour  is  doubtless  more  similar  to 
what  takes  place  in  our  atmosphere.  The  blue,  more- 
over, is  far  purer  and  more  sky-like  than  that  obtained 
from  Briicke's  turbid  meai^m.  Never,  even  in  the 
skies  of  the  Alps,  have  I  seen  a  richer  or  a  purer  blue 
than  that  attainable  by  a  suitable  disposition  of  the 
light  falling  upon  the  precipitated  vapour. 

Iodide  of  AllyL — Among  the  liquids  hitherto  sub- 
jected to  the  concentrated  electric  light,  iodide  of  allyl, 
in  point  of  rapidity  and  intensity  of  action,  comes  next 
to  the  nitrite  of  amyl.  With  the  iodide  I  have  em- 
ployed both  oxygen  and  hydrogen,  as  well  as  air,  as  a 
vehicle,  and  found  the  effect  in  all  cases  substantially 
the  same.  The  cloud-column  here  was  exquisitely 
beautiful.  It  revolved  round  the  axis  of  the  decom- 
posing beam ;  it  was  nipped  at  certain  places  like  an 
hour-glass,  and  round  the  two  bells  of  the  glass  delicate 
cloud-filaments  twisted  themselves  in  spirals.  It  also 
folded  itself  into  convolutions  res«^mbling  those  of  shells. 
In  certain  conditions  of  the  atmosphere  in  the  Alps  I 
have  often  observed  clouds  of  a  special  pearly  lustre ; 
when  hydrogen  was  made  the  vehicle  of  the  iodide-of- 
allyl  vapour  a  similar  lustre  was  most  exquisitely  shown. 


DECOMPOSITION  BY  LIGHT.  107 

With  a  suitable  disposition  of  the  light,  the  purple  hue 
of  iodine-vapour  came  out  very  strongly  in  the  tube. 

The  remark  already  made,  as  to  the  bearing  of  the 
decomposition  of  nitrite  of  amyl  by  light  on  the 
question  of  molecular  absorption,  applies  here  also  ;  for 
were  the  absorption  the  work  of  the  molecule  as  a  whole, 
the  iodine  would  not  be  dislodged  from  the  allyl  with 
which  it  is  combined.  The  non-synchronism  of  iodine 
with  the  waves  of  obscure  heat  is  illustrated  by  its  mar- 
vellous transparency  to  such  heat.  May  not  its  syn- 
chronism with  the  waves  of  light  in  the  present  instance 
be  the  cause  of  its  divorce  from  the  allyl  ? 

Iodide  oflsopropyl. — The  action  of  light  upon  the 
vapour  of  this  liquid  is,  at  first,  more  languid  than  upon 
iodide  of  allyl ;  indeed  many  beautiful  reactions  may 
be  overlooked,  in  consequence  of  this  languor  at  the 
commencement.  After  some  minutes'  exposure,  how- 
ever, clouds  begin  to  form,  which  grow  in  density  and  in 
beauty  as  the  light  continues  to  act.  In  every  experi- 
ment hitherto  made  with  this  substance  the  column  of 
cloud  filling  the  experimental  tube,  was  divided  into 
two  distinct  parts  near  the  middle  of  the  tube.  In  one 
experiment  a  globe  of  cloud  formed  at  the  centre,  from 
which,  right  and  left,  issued  an  axis  uniting  the  globe 
with  two  adjacent  cylinders.  Both  globe  and  cylinders 
were  animated  by  a  common  motion  of  rotation.  As 
the  action  continued,  paroxysms  of  motion  were  mani- 
fested ;  the  various  parts  of  the  cloud  would  rush  through 
each  other  with  sudden  violence.  During  these  motions 
beautiful  and  grotesque  cloud-forms  were  developed. 
At  some  places  the  nebulous  mass  would  become  ribbed 
so  as  to  resemble  the  graining  of  wood ;  a  longitudinal 
motion  would  at  times  generate  in  it  a  series  of  curved 
transverse  bands,  the  retarding  influence  of  the  sides 
of  thd  tube  causing  an  appearance  resembling,  on  a 


108  FRAGMENTS  OF  SCIENCE. 

small  scale,  the  dirt-bands  of  the  Mer  de  Grlace.  In  the 
anterior  portion  of  the  tube  those  sudden  commotions 
were  most  intense;  here  buds  of  cloud  would  sprout 
forth,  and  grow  in  a  few  seconds  into  perfect  flower-like 
forms.  The  cloud  of  iodide  of  isopropyl  had  a  character 
of  its  own,  and  differed  materially  from  all  others  that  I 
had  seen.  A  gorgeous  mauve  colour  was  observed  in 
the  last  twelve  inches  of  the  tube  ;  the  vapour  of  iodine 
was  present,  and  it  may  have  been  the  sky-blue  scat- 
tered by  the  precipitated  particles  which,  mingling  with 
the  purple  of  the  iodine,  produced  the  mauve.  As  in 
all  other  cases  here  adduced,  the  effects  were  proved  to 
be  due  to  the  light ;  they  never  occurred  in  darkness. 

The  forms  assumed  by  some  of  those  aclinic  clouds, 
as  I  propose  to  call  them,  in  consequence  uf  rotations 
and  other  motions,  due  to  ^ differences  of  temperature, 
are  perfectly  astounding.  I  content  myself  here  with  a 
meagre  description  of  one  more  of  them. 

The  tube  being  filled  with  the  sensitive  mixture,  the 
beam  was  sent  through  it,  the  lens  at  the  same  time 
being  so  placed  as  to  produce  a  cone  of  very  intense 
light.  Two  minutes  elapsed  before  anything  was 
visible ;  but  at  the  end  of  this  time  a  faint  bluish 
cloud  appeared  to  hang  itself  on  the  most  concentrated 
portion  of  the  beam. 

Soon  afterwards  a  second  cloud  was  formed  five 
inches  farther  down  the  experimental  tube.  Both 
clouds  were  united  by  a  slender  cord  of  the  same  bluish 
tint  as  themselves. 

As  the  action  of  the  light  continued,  the  first  cloud 
gradually  resolved  itself  into  a  series  of  parallel  disks  of 
exquisite  delicacy,  which  rotated  round  an  axis  perpen- 
dicular to  their  surfaces,  and  finally  blended  to  a  screw 
surface  with  an  inclined  generatrix.  This  gradually 
changed  into  a  filmy  funnel,  from  the  narrow  end  of 
which  the  '  cord '  extended  to  the  cloud  in  advance. 


ARTIFICIAL  SKY.  109 

The  latter  also  underwent  slow  but  incessant  modifica- 
tion. It  first  resolved  itself  into  a  series  of  strata  re- 
sembling those  of  the  electric  discharge.  After  a  little 
time,  and  through  changes  which  it  was  diflBcult  to 
follow,  both  clouds  presented  the  appearance  of  a  seriea 
of  concentric  funnels  set  one  within  the  other,  the 
interior  ones  being  seen  through  the  outer  ones.  Those 
of  the  distant  cloud  resembled  claret-glasses  in  shape. 
As  many  as  six  funnels  were  thus  concentrically  set 
together,  the  two  series  being  united  by  the  delicate 
cord  of  cloud  already  referred  to.  Other  cords  and 
slender  tubes  were  afterwards  formed,  which  coiled 
themselves  in  delicate  spirals  around  the  funnels. 

Eendering  the  light  along  the  connecting-cord  more 
intense,  it  diminished  in  thickness  and  became  whiter ; 
this  was  a  consequence  of  the  enlargement  of  its  par- 
ticles. The  cord  finally  disappeared,  while  the  funnels 
melted  into  two  ghost-like  films,  shaped  like  parasols. 
They  were  barely  visible,  being  of  an  exceedingly  deli- 
cate blue  tint.  They  seemed  woven  of  blue  air.  To  com- 
pare them  with  cobweb  or  with  gauze  would  be  to  liken 
them  to  something  infinitely  grosser  than  themselves. 

In  all  cases  a  distant  candle-flame,  when  looked  at 
through  the  cloud,  was  sensibly  undimmed. 


§  2.  On  the  Blue  Colour  of  the  Sky,  and  the 
Polarisation  of  Skylight.^ 

1869. 

After  the  communication  to  the  Royal  Society  of 

the  foregoing  brief  account  of  a  new  Series  of  Chemical 

Reactions  produced  by  Light,  the  experiments  upon 

*  In  my  *  Lectures  on  Light '  (Longmans),  the  polarisation  of 
light  will  be  found  briefly,  but,  I  trust,  clearly  explained. 


no  FEAGMENTS  OF  SCIENCE. 

this  subject  were  continued,  the  number  of  substances 
thus  acted  on  being  considerably  increased. 

I  now,  however,  beg  to  direct  attention  to  two  ques- 
tions glanced  at  incidentally  in  the  preceding  pages — the 
blue  colour  of  the  sky,  and  the  polarisation  of  skylight. 
Reserving  the  historic  treatment  of  the  subject  for  a 
more  fitting  occasion,  I  would  merely  mention  now  ,that 
these  questions  constitute,  in  the  opinion  of  our  most 
eminent  authorities,  the  two  great  standing  enigmas  of 
meteorology.  Indeed  it  was  the  interest  manifested  in 
them  by  Sir  John  Herschel,  in  a  letter  of  singular 
speculative  power,  addressed  to  myself,  that  caused  me 
to  enter  upon  the  consideration  of  these  questions  so 
soon. 

The  apparatus  with  which  I  work  consists,  as  already 
stated,  of  a  glass  tube  about  a  yard  in  length,  and  from 
2-^  to  3  inches  internal  diameter.  The  vapour  to  be 
examined  is  introduced  into  this  tube  in  the  manner 
already  described,  and  upon  it  the  condensed  beam  of 
the  electric  lamp  is  permitted  to  act,  until  the  neutrality 
or  the  activity  of  the  substance  has  been  declared. 

It  has  hitherto  been  my  aim  to  render  the  chemical 
action  of  light  upon  vapours  visible.  For  this  purpose 
substances  have  been  chosen,  one  at  least  of  whose  pro- 
ducts of  decomposition  under  light  shall  have  a  boiling- 
point  so  high,  that  as  soon  as  the  substance  is  formed 
it  shall  be  precipitated.  By  graduating  the  quantity 
of  the  vapour,  this  precipitation  may  be  rendered  of 
any  degree  of  fineness,  forming  particles  distinguishable 
by  the  naked  eye,  or  far  beyond  the  reach  of  our  highest 
microscopic  powers.  I  have  no  reason  to  doubt  that 
particles  may  be  thus  obtained,  whose  diameters  con- 
stitute but  a  small  fraction  of  the  length  of  a  wave  of 
violet  light. 

In  all  cases  when  the  vapours  of  the  liquids  em- 


AETIFICIAL  SKY.  Ill 

ployed  are  sufficiently  attenuated,  no  matter  what  the 
liquid  may  be,  the  visible  action  commences  with  the 
formation  of  a  blue  cloud.  But  here  I  must  g^ard 
myself  against  all  misconception  as  to  the  use  of  this 
term.  The  '  cloud '  here  referred  to  is  totally  invisible 
in  ordinary  daylight.  To  be  seen,  it  requires  to  be 
surrounded  by  darkness,  it  only  being  illuminated  by  a 
powerful  beam  of  light.  This  blue  cloud  differs  in 
many  important  particulars  from  the  finest  ordinary 
clouds,  and  might  justly  have  assigned  to  it  an  inter- 
mediate position  between  such  clouds  and  true  vapour. 
With  this  explanation,  the  term  '  cloud,'  or  '  incipient 
cloud,'  or  'actinic  cloud,'  as  I  propose  to  employ  it, 
cannot,  I  think,  be  misunderstood. 

I  had  been  endeavouring  to  decompose  carbonic 
acid  gas  by  light.  A  faint  bluish  cloud,  due  it  may  be, 
or  it  may  not  be,  to  the  residue  of  some  vapour  pre- 
viously employed,  was  formed  in  the  experimental  tube 
On  looking  across  this  cloud  through  a  Ni col's  prism, 
the  line  of  vision  being  horizontal,  it  was  foimd  that 
when  the  short  diagonal  of  the  prism  was  vertical,  the 
quantity  of  light  reaching  the  eye  was  greater  than 
when  the  long  diagonal  was  vertical.  When  a  plate  of 
tourmaline  was  held  between  the  eye  and  the  bluish 
cloud,  the  quantity  of  light  reaching  the  eye  when  the 
axis  of  the  prism  was  perpendicular  to  the  axis  of  the 
illuminating  beam,  was  greater  than  when  the  axes  of 
the  crystal  and  of  the  beam  were  parallel  to  each  other. 

This  was  the  result  all  round  the  experimental  tube. 
Causing  the  crystal  of  tourmaline  to  revolve  round  the 
tube,  with  its  axis  perpendicular  to  the  illuminating 
beam,  the  quantity  of  light  that  reached  the  eye  was  in 
all  its  positions  a  maximum.  When  the  crystallographic 
axis  was  parallel  to  the  axis  of  the  beam,  the  quantity 
of  light  transmitted  by  the  crystal  was  a  minimum. 


112  FRAGMENTS  OF  SCIENCE. 

From  the  illuminated  bluish  cloud,  therefore,  polarised 
light  was  discharged,  the  direction  of  maximum  polari- 
sation being  at  right  angles  to  the  illuminating  beam ; 
the  plane  of  vibration  of  the  polarised  light  was  per- 
pendicular to  the  beam.^ 

Thin  plates  of  selenite  or  of  quartz,  placed  between 
the  Nicol  and  the  actinic  cloud,  displayed  the  colours  of 
polarised  light,  these  colours  being  most  vivid  when  the 
line  of  vision  was  at  right  angles  to  the  experimental 
tube.  The  plate  of  selenite  usually  employed  was  a 
circle,  thinnest  at  the  centre,  and  augmenting  uniformly 
in  thickness  from  the  centre  outwards.  When  placed 
in  its  proper  position  between  the  Nicol  and  the  cloud, 
it  exhibited  a  system  of  splendidly-coloured  rings. 

The  cloud  here  referred  to  was  the  first  operated 
upon  in  the  manner  described.  It  may,  however,  be 
greatly  improved  upon  by  the  choice  of  proper  substances, 
and  by  the  application,  in  proper  quantities,  of  the  sub- 
stances chosen.  Benzol,  bisulphide  of  carbon,  nitrite  of 
amyl,  nitrite  of  butyl,  iodide  of  allyl,  iodide  of  isopropyl, 
and  many  other  substances  may  be  employed,  I  will 
take  the  nitrite  of  butyl  as  illustrative  of  the  means 
adopted  to  secure  the  best  result,  with  reference  to  the 
present  question. 

And  here  it  may  be  mentioned  that  a  vapour,  which 
when  alone,  or  mixed  with  air  in  the  experimental  tube, 
resists  the  action  of  light,  or  shows  but  a  feeble  result 
of  this  action,  may,  when  placed  in  proximity  with 
another  gas  or  vapour,  exhibit  vigorous,  if  not  violent 
action.  The  case  is  similar  to  that  of  carbonic  acid 
gas,  which,  diffused  in  the  atmosphere,  resists  the  de- 

•  This  is  still  an  undecided  point ;  but  the  probabilities  are  so 
much  in  its  favour,  and  it  is  in  my  opinion  so  much  preferable  to 
have  a  physical  image  on  which  the  mind  can  rest,  that  I  do  not 
hesitate  to  employ  the  phraseology  in  the  text. 


ARTIFICIAL  SKY.  113 

composing  action  of  solar  light,  but  when  placed  in 
contiguity  with  chlorophyl  in  the  leaves  of  plants,  has 
its  molecules  shaken  asunder. 

Dry  air  was  permitted  to  bubble  through  the  liquid 
nitrite  of  butyl,  until  the  experimental  tube,  which  had 
been  previously  exhausted,  was  filled  with  the  mixed 
air  and  vapour.  The  visible  action  of  light  upon  the 
mixture  after  fifteen  minutes'  exposure  was  slight. 
The  tube  was  afterwards  filled  with  half  an  atmosphere 
of  the  mixed  air  and  vapour,  and  a  second  half-atmo- 
sphere of  air  which  had  been  permitted  to  bubble 
through  fresh  commercial  hydrochloric  acid.  On  send- 
ing the  beam  through  this  mixture,  the  tube,  for  a 
moment,  was  optically  empty.  But  the  pause  amounted 
only  to  a  small  fraction  of  a  second,  a  dense  cloud  being 
immediately  precipitated  upon  the  beam. 

This  cloud  began  blue,  but  the  advance  to  whiteness 
was  so  rapid  as  almost  to  justify  the  application  of  the 
term  instantaneous.  The  dense  cloud,  looked  at  per- 
pendicularly to  its  axis,  showed  scarcely  any  signs  of 
polarisation.  Looked  at  obliquely  the  polarisation  was 
strong. 

The  experimental  tube  being  again  cleansed  and 
exhausted,  the  mixed  air  and  nitrite-of-butyl  vapour 
was  permitted  to  enter  it  until  the  associated  mercury 
column  was  depressed  -^  of  an  inch.  In  other  words, 
the  air  and  vapour,  united,  exercised  a  pj-essure  not 
exceeding  -3-^  of  an  atmosphere.  Air,  passed  through 
a  solution  of  hydrochloric  acid,  was  then  added,  till  the 
mercury  column  was  depressed  three  inches.  The  con- 
densed beam  of  the  electric  light  was  passed  for  some 
time  through  this  mixture  without  revealing  anything 
within  the  tube  competent  to  scatter  the  light.  Soon, 
however,  a  superbly  blue  cloud  was  formed  along  the 
track  of  the  beam,  and  it  continued  blue  sufficiently 


114  FRAGMENTS  OF  SCIENCE. 

long^  to  permit  of  its  thorough  examination.  The  light 
discharged  from  the  cloud,  at  right  angles  to  its  own 
length,  was  at  first  perfectly  polarised.  It  could  be 
totally  quenched  by  the  Nicol.  By  degrees  the  cloud 
became  of  whitish  blue,  and  for  a  time  the  selenite 
colours,  obtained  by  looking  at  it  normally,  were  ex- 
ceedingly brilliant.  The  direction  of  maximum  polari- 
sation was  distinctly  at  right  angles  to  the  illuminating 
beam.  This  continued  to  be  the  case  as  long  as  the 
cloud  maintained  a  decided  blue  colour,  and  even  for 
some  time  after  the  blue  had  changed  to  whitish  blue. 
But,  as  the  light  continued  to  act,  the  cloud  became 
coarser  ,and  whiter,  particularly  at  its  centre,  where  it 
at  length  ceased  to  discharge  polarised  light  in  the 
direction  of  the  perpendicular,  while  it  continued  to  do 
so  at  both  ends. 

But  the  cloud  which  had  thus  ceased  to  polarise  the 
light  emitted  normally,  showed  vivid  selenite  colours 
when  looked  at  obliquely,  proving  that  the  direction  of 
maximum  polarisation  changed  with  the  texture  of  the 
cloud.  This  point  shall  recei^^e  further  illustration 
subsequently. 

A  blue,  equally  rich  and  more  durable,  was  obtained 
by  employing  the  nitrite-of-butyl  vapour  in  a  still  more 
attenuated  condition.  The  instance  here  cited  is  re- 
presentative. In  all  cases,  and  with  all  substances,  the 
cloud  formed  at  the  commencement,  when  the  preci- 
pitated particles  are  sufficiently  fine,  is  blue,  and  it  can 
be  made  to  display  a  colour  rivalling  that  of  the  purest 
Italian  sky.  In  all  cases,  moreover,  this  fine  blue  cloud 
polarises  perfectly  the  beam  which  illuminates  it,  the 
direction  of  polarisation  enclosing  an  angle  of  90°  with 
the  axis  of  the  illuminating  beam. 

It  is  exceedingly  interesting  to  observe  both  the 
perfection  and  the  decay  of  this  polarisation.     For  ten 


AETinCIAL  SKY.  115 

or  fifteen  minutes  after  its  first  appearance  the  light 
from  a  vividly  iUuminated  actinic  cloud,  looked  at 
perpendicularly,  is  absolutely  quenched  by  a  Nicol's 
prism  with  its  longer  diagonal  vertical.  But  as  the 
sky-blue  is  gradually  rendered  impure  by  the  growth  of 
the  particles — in  other  words,  as  real  clouds  begin  to  be 
formed — the  polarisation  begins  to  decay,  a  portion  of 
the  light  passing  through  the  prism  in  all  its  positions. 
It  is  worthy  of  note,  that  for  some  time  after  the  cessa- 
tion of  perfect  polarisation,  the  residual  light  which 
passes,  when  the  Nicol  is  in  its  position  of  minimum 
transmission,  is  of  a  gorgeous  blue,  the  whiter  light  of 
the  cloud  being  extinguished.^  When  the  cloud  texture 
has  become  sufficiently  coarse  to  approximate  to  that 
of  ordinary  clouds,  the  rotation  of  the  Nicol  ceases  to 
have  any  sensible  effect  on  the  quantity  of  light  dis- 
charged normally. 

The  perfection  of  the  polarisation,  in  a  direction 
perpendicular  to  the  illuminating  beam,  is  also  illus- 
trated by  the  following  experiment :  A  Nicol's  prism, 
large  enough  to  embrace  the  entire  beam  of  the  electric 
lamp,  was  placed  between  the  lamp  and  the  experi- 
mental tube.  A  few  bubbles  of  air,  carried  through 
the  liquid  nitrite  of  butyl,  were  introduced  into  the 
tube,  and  they  were  followed  by  about  three  inches 
(measured  by  the  mercurial  gauge)  of  air  which  had 
passed  through  aqueous  hydrochloric  acid.  Sending 
the  polarised  beam  through  the  tube,  I  placed  myself 
in  front  of  it,  my  eye  being  on  a  level  with  its  axis,  my 
assistant  occupying  a  similar  position  behind  the  tube. 
The  short  diagonal  of  the  large  Nicol  was  in  the  first 
instance  vertical,  the  plane  of  vibration  of  the  emergent 

*  This  shows  that  particles  too  large  to  polarise  the  blue,  polar- 
tae  perfectly  light  of  lower  refrangibility. 


116  FEAGMENTS  OF  SCIENCE. 

beam  being  therefore  also  vertical.  As  the  light  con- 
tinued to  act,  a  superb  blue  cloud,  visible  to  both  my 
assistant  and  myself,  was  slowly  formed.  But  this 
cloud,  so  deep  and  rich  when  looked  at  from  the  posi- 
tions mentioned,  utterly  disaj)'peared  when  looked  at 
vertically  downwards^  or  vertically  upwards.  Eeflec- 
tion  from  the  cloud  was  not  possible  in  these  directions. 
When  the  large  Nicol  was  slovvly  turned  round  its  axis, 
the  eye  of  the  observer  being  on  the  level  of  the  beam, 
and  the  line  of  vision  perpendicular  to  it,  entire  extinc- 
tion of  the  light  emitted  horizontally  occurred  when 
the  longer  diagonal  of  the  large  Nicol  was  vertical. 
But  now  a  vivid  blue  cloud  was  seen  when  looked  at 
downwards  or  upwards.  This  truly  fine  experiment, 
which  I  contemplated  making  on  my  own  account,  was 
first  definitely  suggested  by  a  remark  in  a  letter  ad- 
dressed to  me  by  Professor  Stokes. 

As  regards  the  polarisation  of  skylight,  the  greatest 
stumbling-block  has  hitherto  been,  that,  in  accordance 
with  the  law  of  Brewster,  which  makes  the  index  of 
refraction  the  tangent  of  the  polarising  angle,  tlie  re- 
flection which  produces  perfect  polarisation  would 
require  to  be  made  in  air  upon  air ;  and  indeed  this 
led  many  of  our  most  eminent  men,  Brewster  himself 
among  the  number,  to  entertain  the  idea  of  aerial 
molecular  reflection.^     I  have,  however,  operated  upon 

'  *  The  cause  of  the  polarisation  is  evidently  a  reflection  of  the 
sun's  light  upon  somefMiig.  The  question  is  on  what?  Were  t]:e 
angle  of  maximum  polarisation  76°,  we  should  look  to  water  or  ice 
as  the  reflecting  body,  however  inconceivable  the  existence  in  a 
cloudless  atmosphere  and  a  hot  summer's  day  of  unevaporated 
molecules  (particles  ?)  of  water.  But  though  we  were  once  of  this 
opinion,  careful  observation  has  satisfied  us  that  90°,  or  there- 
abouts, is  the  correct  angle,  and  that  therefore  whatever  be  the 
body  on  which  the  light  has  been  reflected,  if  polarised  by  a  singU 
rejieationy  the  polarising  angle  must  be  45°,  and  the  index  of  refrac- 
tion, which  is  the  tangent  of  that  angle,  unity ;  in  other  words,  the 


ARTIFICIAL  SKY.  117 

substances  of  widely  different  refractive  indices,  and 
therefore  of  very  different  polarising  angles  as  ordi- 
narily defined,  but  the  polarisation  of  the  beam,  by  the 
incipient  cloud,  has  thus  far  proved  itself  to  be  abso- 
lutely independent  of  the  polarising  angle.  The  law 
of  Brewster  does  not  apply  to  matter  in  this  condition, 
and  it  rests  with  the  undulatory  theory  to  explain  why. 
Whenever  the  precipitated  particles  are  sufficiently  fine, 
no  matter  what  the  substance  forming  the  particles 
may  be,  the  direction  of  maximum  polarisation  is  at 
right  angles  to  the  illuminating  beam,  the  polaxising 
angle  for  matter  in  this  condition  being  invariably  45°. 
Suppose  our  atmosphere  surrounded  by  an  envelope 
impervious  to  light,  but  with  an  aperture  on  the  sun- 
ward side  through  which  a  parallel  beam  of  solar  light 
could  enter  and  traverse  the  atmosphere.  Surrounded 
by  air  not  directly  illuminated,  the  track  of  such  a 
beam  would  resemble  that  of  the  parallel  beam  of 
the  electric  lamp  through  an  incipient  cloud.  The 
sunbeam  would  be  blue,  and  it  would  discharge  later- 
ally light  in  precisely  the  same  condition  as  that  dis- 
charged by  the  incipient  cloud.  In  fact,  the  azure 
revealed  by  such  a  beam  would  be  to  all  intents  and 
purposes  that  which  I  have  called  a  '  blue  cloud.'  Con- 
versely our  '  blue  cloud '  is,  to  all  intents  and  purposes, 
an  a/rtificial  shy} 

reflection  would  require  to  be  made  in  air  v^on  air  I  *  (Sir  John 
Herschel,  *  Meteorology,'  par.  233.) 

Any  particles,  if  small  enough,  will  produce  both  the  colour  and 
the  polarisation  of  the  sky.  But  is  the  existence  of  small  water- 
particles  on  a  hot  summer's  day  in  the  higher  regions  of  our  atmo- 
iph&re  inconceivable  ?  It  is  to  be  remembered  that  the  oxygen  and 
nitrogen  of  the  air  behave  as  a  vacuum  to  radiant  heat,  the  exceed- 
ingly attenuated  vapour  of  the  higher  atmosphere  being  therefore 
in  practical  contact  with  the  cold  of  space. 

•  The  opinion  of  Sir  John  Herschel,  connecting  the  polarisation 
and  the  blue  colour  of  the  sky,  is  verified  by  the  foregoing  results 


118  FEAGMENTS  OF  SCIENCE. 

But,  as  regards  the  polarisation  of  the  sky,  we  kno"W 
that  not  only  is  the  direction  of  maximum  polarisation 
at  right  angles  to  the  track  of  the  solar  beams,  but  that 
at  certain  angular  distances,  probably  variable  ones,  from 
the  sun,  '  reutral  points,*  or  points  of  no  polarisation, 
exist,  on  both  sides  of  which  the  planes  of  atmospheric 
polarisation  are  at  right  angles  to  each  other.  I  have 
made  various  observations  upon  this  subject  which  are 
reserved  for  the  present ;  but,  pending  the  more  com- 
plete examination  of  the  question,  the  following  facts 
bearing  upon  it  may  be  submitted. 

The  parallel  beam  employed  in  these  experiments 
tracked  its  way  through  the  laboratory  air,  exactly  as 
sunbeams  are  seen  to  do  in  the  dusty  air  of  London.  I 
have  reason  to  believe  that  a  great  portion  of  the  matter 
thus  floating  in  the  laboratory  air  consists  of  organic 
particles,  which  are  capable  of  imparting  a  perceptibly 
bluish  tint  to  the  air.  These  also  showed,  though  far 
less  vividly,  all  the  effects  of  polarisation  obtained  with 
the  incipient  clouds.  The  light  discharged  laterally 
from  the  track  of  the  illuminating  beam  was  polarised, 
though  not  perfectly,  the  direction  of  maximum  polar- 
isation being  at  right  angles  to  the  beam.  At  all  points 
of  the  beam,  moreover,  throughout  its  entire  length, 
the  light  emitted  normally  was  in  the  same  state  of 
polarisation.  Keeping  the  positions  of  the  Nicol  and 
the  selenite  constant,  the  same  colours  were  observed 

*  The  more  the  subject  [the  polarisation  of  skylight]  is  considered,' 
writes  this  eminent  philosopher,  *  the  more  it  will  be  found  beset 
with  difficulties,  and  its  explanation  when  arrived  at  will  probably 
be  found  to  carry  with  it  that  of  the  blue  colour  of  the  sky  itself, 
and  of  the  great  quantity  of  light  it  actually  does  send  down  to  us.' 

*  We  may  observe,  too,'  he  adds,  *  that  it  is  only  where  the  purity 
of  the  sky  is  most  absolute  that  the  polarisation  is  developed  in 
its  highest  degree,  and  that  where  there  is  the  slightest  perceptible 
tendency  to  cirrus  it  is  materially  impaired.'  This  applies  word 
for  word  to  our  *  incipient  clouds.' 


AETIFICIAL  SKY.  119 

througliout  the  entire  beam,  when  the  line  of  vision 
was  perpendicular  to  its  length. 

The  horizontal  column  of  air,  thus  illuminated,  was 
18  feet  long,  and  could  therefore  be  looked  at  very  ob- 
liquely. I  placed  myself  near  the  end  of  the  beam,  as 
it  issued  from  the  electric  lamp,  and,  looking  through 
the  Nicol  and  selenite  more  and  more  obliquely  at  the 
beam,  observed  the  colours  fading  until  they  disappeared. 
Augmenting  the  obliquity  the  colours  appeared  once 
more,  but  they  were  now  complementary  to  the  former 
ones. 

Hence  this  beam,  like  the  sky,  exhibited  a  neutral 
point,  on  opposite  sides  of  which  the  light  was  polarised 
in  planes  at  right  angles  to  each  other. 

Thinking  that  the  action  observed  in  the  laboratory 
might  be  caused,  in  some  way,  by  the  vaporous  fumes 
diffused  in  its  air,  I  had  the  light  removed  to  a  room  at 
the  top  of  the  Eoyal  Institution.  The  track  of  the 
beam  was  seen  very  finely  in  the  air  of  this  room,  a 
length  of  14  or  15  feet  being  attainable.  This  beam 
exhibited  all  the  effects  observed  with  the  beam  in  the 
laboratory.  Even  the  uncondensed  electric  light  falling 
on  the  floating  matter  showed,  though  faintly,  the  effects 
of  polarisation. 

When  the  air  was  so  sifted  as  to  entirely  remove  the 
visible  floating  matter,  it  no  longer  exerted  any  sensible 
action  upon  the  light,  but  behaved  like  a  vacuum.  The 
light  is  scattered  and  polarised  by  jparticles^  not  by 
molecules  or  atoms. 

By  operating  upon  the  fumes  of  chloride  of  ammo- 
nium, the  smoke  of  brown  paper,  and  tobacco-smoke,  I 
had  varied  and  confirmed  in  many  ways  those  experi- 
ments on  neutral  points,  when  my  attention  was  drawn 
by  Sir  Charles  Wheatstone  to  an  important  observation 
communicated  to  the  Paris  Academy  in  1860  by  Pro- 
9 


120  FEAaMENTS  OF  SCIENCE. 

fessor  Govi,  of  Turin.'  M.  Govi  bad  been  led  to  examine 
a  beam  of  light  sent  through  a  room  in  which  were  iiuc- 
cessively  diffused  the  smoke  of  incense,  and  tobacco- 
smoke.  His  first  brief  communication  stated  the  fact 
of  polarisation  by  such  smoke  ;  but  in  his  second  com- 
munication he  announced  the  discovery  of  a  neutral 
point  in  the  beam,  at  the  opposite  sides  of  which  the 
light  ,was  polarised  in  planes  at  right  angles  to  each 
other. 

But  unlike  my  observations  on  the  laboratory  air, 
and  unlike  the  action  of  the  sky,  the  direction  of  maxi- 
mum polarisation  in  M.  Grovi's  experiment  enclosed  a 
very  small  angle  with  the  axis  of  the  illuminating  beam. 
The  question  was  left  in  this  condition,  and  I  am  not 
aware  that  M.  Govi  or  any  other  investigator  has  pur- 
sued it  fuither. 

I  had  noticed,  as  before  stated,  that  as  the  clouds 
formed  in  the  experimental  tube  became  denser,  the 
polarisation  of  the  light  discharged  at  right  angles  to 
the  beam  became  weaker,  the  direction  of  maximum 
polarisation  becoming  oblique  to  the  beam.  Experi- 
ments on  the  fumes  of  chloride  of  ammonium  gave  me 
also  reason  to  suspect  that  the  position  of  the  neutral 
point  was  not  constant,  but  that  it  varied  with  the 
density  of  the  illuminated  fumes. 

The  examination  of  these  questions  led  to  the  follow- 
ing new  and  remarkable  results  :  The  laboratory  being 
well  filled  with  the  fumes  of  incense,  and  sufficient  time 
being  allowed  for  their  uniform  diffusion,  the  electric 
beam  was  sent  through  the  smoke.  From  the  track  of 
the  beam  polarised  light  was  discharged  ;  but  the  direc- 
tion of  maximum  polarisation,  instead  of  being  perpen- 
dicular, now  enclosed  an  angle  of  only  12°  or  13°  with 
the  axis  of  the  beam. 

^  *Comptes  Rendus,'  tome  li.  pp.  360  and  669, 


AKTIFICIAL  SKY.  121 

A  neutral  point,  with  complementary  effects  at  oppo- 
site sides  of  it,  was  also  exhibited  by  the  beam.  The 
angle  enclosed  by  the  axis  of  the  beam,  and  a  line  drawn 
from  the  neutral  point  to  the  observer's  eye,  measured 
in  the  first  instance  66°, 

The  windows  of  the  laboratory  were  now  opened  for 
some  minutes,  a  portion  of  the  incense-smoke  being  per- 
mitted to  escape.  On  again  darkening  the  room  and 
turning  on  the  light,  the  line  of  vision  to  the  neutral 
point  was  found  to  enclose,  with  the  axis  of  the  beam, 
an  angle  of  63°. 

The  windows  were  again  opened  for  a  few  minutes, 
more  of  the  smoke  being  permitted  to  escape.  Measured 
as  before,  the  angle  referred  to  was  found  to  be  54°. 

This  process  was  repeated  three  additional  times ; 
the  neutral  point  was  found  to  recede  lower  and  lower 
down  the  beam,  the  angle  between  a  line  drawn  from 
the  eye  to  the  neutral  point  and  the  axis  of  the  beam 
falling  successively  from  54°  to  49°,  43°  and  33°. 

The  distances,  roughly  measured,  of  the  neutral 
point  from  the  lamp,  corresponding  to  the  foregoing 
Beries  of  observations,  were  these : — 


1st  observation 

.    2  feel 

.    2  inches. 

2nd         „ 

.    2    „ 

6      „ 

3rd         „ 

.     2    „ 

10     „ 

4th 

.    3    „ 

2      „ 

5th 

.     3    „ 

7     „ 

6th         „ 

.    4    „ 

6      „ 

At  the  end  of  this  series  of  experiments  the  direction 
of  maximum  polarisation  had  again  become  normal  to 
the  beam. 

The  laboratory  was  next  filled  with  the  fumes  of 
gunpowder.  In  five  successive  experiments,  correspond- 
ing to  five  diiferent  densities  of  the  gunpowder-smoke, 
the  angles  enclosed  between  the  line  of  vision  to  the 


122  FEAGMENTS  OF  SCIENCE. 

neutral  point  and  the  axis  of  the  beam,  were  63°,  50**, 
47°,  42°,  and  38°  respectively. 

After  the  clouds  of  gunpowder  had  cleared  away,  the 
laboratory  was  filled  with  the  fumes  of  common  resin, 
rendered  so  dense  as  to  be  very  irritating  to  the  lungs. 
The  direction  of  maximum  polarisation  enclosed,  in  this 
case,  an  angle  of  12°,  or  thereabouts,  with  the  axis  of 
the  beam.  Looked  at,  as  in  the  former  instances,  from 
a  position  near  the  electric  lamp,  no  neutral  point  was 
observed  throughout  the  entire  extent  of  the  beam. 

When  this  beam  was  looked  at  normally  through  the 
selenite  and  Nicol,  the  ring-system,  though  not  brilliant, 
was  distinct.  Keeping  the  eye  upon  the  plate  of  sele- 
nite, and  the  line  of  vision  perpendicular,  the  windows 
were  opened,  the  blinds  remaining  undrawn.  The  resi- 
nous fumes  slowly  diminished,  and  as  they  did  so  the 
ring-system  became  paler.  It  finally  disappeared. 
Continuing  to  look  in  the  same  direction,  the  rings  re- 
vived, but  now  the  colours  were  complementary  to  the 
former  ones.  The  neutral  'point  had  passed  me  in  its 
motion  down  the  beam,  consequent  upon  the  attenua- 
tion of  the  fumes  of  resin, 

"With  the  fumes  of  chloride  of  ammonium  substan- 
tially the  same  results  were  obtained.  Sufficient,  how- 
ever, has  been  here  stated  to  illustrate  the  variability 
of  the  position  of  the  neutral  point.^ 

By  a  puff  of  tobacco-smoke,  or  of  condensed  steam, 
blown  into  the  illuminated  beam,  the  brilliancy  of  the 
selenite  colours  may  be  greatly  enhanced.  But  with 
different  clouds  two  different  effects  are  produced.  Let 
the  ring-system  observed  in  the  common  air  be  brought 
to  its  maximum  strength,  and  then  let  an  attenuated 

>  Brewster  has  proved  the  variability  of  the  position  of  the 
neutral  point  for  skylight  with  the  sun's  altitude,  a  result  obviously 
oonnected  with  the  foregoing  experiments. 


AKTTFICIAL  SKY.  •  123 

cloud  of  chloride  of  ammonium  be  thrown  into  the 
beam  at  the  point  looked  at ;  the  ring  system  flashes 
out  with  augmented  brilliancy,  but  the  character  of  the 
polarisation  remains  unchanged.  This  is  also  the  case 
when  phosphorus,  or  sulphur,  is  burned  underneath  the 
beam,  so  as  to  cause  the  fine  particles  of  phosphorus 
or  of  sulphur  to  rise  into  the  light.  With  the  sul- 
phur-fumes the  brilliancy  of  the  colours  is  exceedingly 
intensified ;  but  in  none  of  these  cases  is  there  any 
change  in  the  character  of  the  polarisation. 

But  when  a  puff  of  the  fumes  of  hydrochloric  acid, 
hydriodic  acid,  or  nitric  acid  is  thrown  into  the  beam, 
there  is  a  complete  reversal  of  the  selenite  tints.  Each 
of  these  clouds  twists  the  plane  of  polarisation  90°, 
causing  the  centre  of  the  ring-system  to  change  from 
black  to  white,  and  the  rings  themselves  to  emit  their 
complementary  colours.^ 

Almost  all  li(|uids  have  motes  in  them  sufficiently 
numerous  to  polarise  sensibly  the  light,  and  very  beau- 
tiftil  effects  may  be  obtained  by  simple  artificial  devices. 
When,  for  example,  a  cell  of  distilled  water  is  placed 
in  front  of  the  electric  lamp,  and  a  thin  slice  of  the 
beam  is  permitted  to  pass  through  it,  scarcely  any 
polarised  light  is  discharged,  and  scarcely  any  colour 
produced  with  a  plate  of  selenite.  But  if  a  bit  of 
soap  be  agitated  in  the  water  above  the  beam,  the 
moment  the  infinitesimal  particles  reach  the  light  the 
liquid  sends  forth  laterally  almost  perfectly  polarised 
light ;  and  if  the  selenite  be  employed,  vivid  colours 
flash  into  existence.     A  still  more  brilliant  result  is 

*  Sir  John  Herschel  suggested  to  me  that  this  change  of  the 
polarisation  from  positive  to  negative  may  indicate  a  change  from 
polarisation  by  reflection  to  polarisation  by  refraction.  This 
thought  repeatedly  occurred  to  me  while  looking  at  the  effects  j 
but  it  will  require  much  following  up  before  it  emerges  into  clear- 
ness. 


124  FEAGMENTS  OF  SCIENCE. 

obtained  with  mastic   dissolved  in  a  great  excess  of 
alcohol. 

The  selenite  rings,  in  fact,  constitute  an  extremely 
delicate  test  as  to  the  collective  quantity  of  individually 
invisible  particles  in  a  liquid.  Commencing  with  dis- 
tilled water,  for  example,  a  thick  slice  of  light  is  neces- 
sary to  make  the  polarisation  of  its  suspended  particlet 
sensible.  A  much  thinner  slice  suffices  for  commor 
water ;  while,  with  Briicke's  precipitated  mastic,  a 
slice  too  thin  to  produce  any  sensible  effect  with  mosi 
other  liquids,  suffices  to  bring  out  vividly  the  selenitft 
colours. 

§  3.    The  Sky  op  the  Alps. 

The  vision  of  an  object  always  implies  a  differential 
action  on  the  retina  of  the  observer.  The  object  is 
distinguished  from  surrounding  space  by  its  excess  or 
defect  of  light  in  relation  to  that  space.  By  altei  ^ng 
the  illumination,  either  of  the  object  itself  or  of  its 
environment,  we  alter  the  appearance  of  the  object. 
Take  the  case  of  clouds  floating  in  the  atmoF7>here  with 
patches  of  blue  between  them.  Anything  that  changes 
the  illumination  of  either  alters  the  appearance  of  both, 
that  appearance  depending,  as  stated,  upon  differential 
action.  Now  the  light  of  the  sky,  being  polarised, 
may,  as  the  reader  of  the  foregoing  pages  knows,  be  in 
great  part  quenched  by  a  Nicol's  prism,  while  the  light 
of  a  common  cloud,  being  unpolarised,  cannot  be  thus 
extinguished.  Hence  the  possibility  of  very  remarkable 
variations,  not  only  in  the  aspect  of  the  firmament, 
which  is  really  changed,  but  also  in  tlie  aspect  of  the 
clouds,  which  have  that  firmament  as  a  background. 
It  is  possible,  for  example,  to  choose  clouds  of  such  a 
depth  of  shade  that  when  the  Nicol  quenches  the  light 
behind  them,  they  shall  vanish,  being  undistinguishable 


AETIFICIAL  SKY.  125 

from  the  residual  dull  tint  which  outlives  the  extinction 
of  the  brilliancy  of  the  sky.  A  cloud  less  deeply 
shaded,  but  still  deep  enough,  when  viewed  with  the 
naked  eye,  to  appear  dark  on  a  bright  ground,  is  sud-  - 
denly  changed  to  a  white  cloud  on  a  dark  ground  by. 
the  quenching  of  the  light  behind  it.  When  a  reddish 
cloud  at  sunset  chances  to  float  in  the  region  of  maxi- 
mum polarisation,  the  quenching  of  the  surrounding 
light  causes  it  to  flash  with  a  brighter  crimson.  Last 
Easter  eve  the  Dartmoor  sky,  which  had  just  been 
cleansed  by  a  snow-storm,  wore  a  very  wild  appearance. 
Hound  the  horizon  it  was  of  steely  brilliancy,  while 
reddish  cumuli  and  cirri  floated  southwards.  When  the 
sky  was  quenched  behind  them  these  floating  masses 
seemed  like  dull  embers  suddenly  blown  upon;  they 
brightened  like  a  fire. 

In  the  Alps  we  have  the  most  magnificent  examples 
of  crimson  clouds  and  snows,  so  that  the  eff'ects  just 
referred  to  may  be  here  studied  under  the  best  possible 
conditions.  On  August  23,  1869,  the  evening  Alpen- 
glow  was  very  fine,  though  it  did  not  reach  its  maximum 
depth  and  splendour.  The  side  of  the  Weisshorn  seen 
from  the  Bel  Alp,  being  turned  from  the  sun,  was  tinted 
mauve ;  but  I  wished  to  observe  one  of  the  rose-coloured 
buttresses  of  the  mountain.  Such  a  one  was  visible 
from  a  point  a  few  hundred  feet  above  the  hotel.  The 
Matterhorn  also,  though  for  the  most  part  in  shade, 
had  a  crimson  projection,  while  a  deep  ruddy  red 
lingered  along  its  western  shoulder.  Four  distinct 
peaks  and  buttresses  of  the  Dom,  in  addition  to  its 
dominant  head — all  covered  with  pure  snow— were 
reddened  by  the  light  of  sunset.  The  shoulder  of  the 
Alphubel  was  similarly  coloured,  while  the  great  masa 
of  the  Fletschorn  was  all  a-glow,  and  so  was  the  snowy 
spine  of  the  Monte  Leone, 


126  FRAGMENTS  OF  SCIENCE. 

Looking  at  the  Weisshom  through  the  Nicol,  the 
glow  of  its  protuherance  was  strong  or  weak  according 
to  the  position  of  the  prism.  The  summit  also  under- 
went striking  changes.  In  one  position  of  the  prism  it 
exhibited  a  pale  white  against  a  dark  background ;  in 
the  rectangular  position  it  was  a  dark  mauve  against  a 
light  background.  The  red  of  the  Matterhorn  changed 
in  a  similar  manner ;  but  the  whole  mountain  also 
passed  through  wonderful  changes  of  definition.  The 
air  at  the  time  was  filled  with  a  silvery  haze,  in  which 
the  Matterhorn  almost  disappeared.  This  could  be  wholly 
quenched  by  the  Nicol,  and  then  the  mountain  sprang 
forth  with  astonishing  solidity  and  detachment  from 
the  surrounding  air.  The  changes  of  the  Dom  wore 
still  more  wonderful.  A  vast  amount  of  light  could  be 
removed  from  the  sky  behind  it,  for  it  occupied  the 
position  of  maximum  polarisation.  By  a  little  practice 
with  the  Nicol  it  was  easy  to  render  the  extinction  of 
the  light,  or  its  restoration,  almost  instantaneous. 
When  the  sky  was  quenched,  the  four  minor  peaks  and 
buttresses,  and  the  summit  of  the  Dom,  together  with 
the  shoulder  of  the  Alphubel,  glowed  as  if  set  suddenly 
on  fire.  This  was  immediately  dimmed  by  turning  the 
Nicol  through  an  angle  of  90°.  It  was  not  the  stoppage 
of  the  light  of  the  sky  behind  the  mountains  alone 
which  produced  this  startling  effect;  the  air  between 
them  and  me  was  highly  opalescent,  and  the  quench- 
ing of  this  intermediate  glare  augmented  remarkably  the 
distinctness  of  the  mountains. 

On  the  morning  of  August  24  similar  effects  were 
finely  shown.  At  10  a.m.  all  three  mountains,  the  Dom, 
the  Matterhorn,  and  the  Weisshorn,  were  powerfully 
affected  by  the  Nicol.  But  in  this  instance  also,  the 
line  drawn  to  the  Dom  being  very  nearly  perpendicular 
to  the  solar  beams,  the  effects  on  this  mountain  were 


ARTIFICIAL  SKY.  127 

most  striking.  The  grey  summit  of  the  Matterhorn, 
at  the  same  time,  could  scarcely  be  distinguished  frcm 
the  opalescent  haze  around  it;  but  when  the  Nicol 
quenched  the  haze,  the  summit  became  instantly  iso- 
lated, and  stood  out  in  bold  definition.  It  is  to  be 
remembered  that  in  the  production  of  these  effects  the 
only  things  changed  are  the  sky  behind,  and  the  lumi- 
nous haze  in  front  of  the  mountains;  that  these  are 
changed  because  the  light  emitted  from  the  sky  and 
from  the  haze  is  plane  polarised  light,  and  that  the 
light  from  the  snows  and  from  the  mountains,  being 
sensibly  unpolarised,  is  not  directly  affected  by  the 
Nicol.  It  will  also  be  understood  that  it  is  not  the 
interposition  of  the  haze  as  an  opaque  body  that  renders 
the  mountains  indistinct,  but  that  it  is  the  light  of 
the  haze  which  dims  and  bewilders  the  eye,  and  thus 
weakens  the  definition  of  objects  seen  through  it. 

These  results  have  a  direct  bearing  upon  what 
artists  call  '  aerial  perspective.'  As  we  look  from  the 
summit  of  Mont  Blanc,  or  from  a  lower  elevation,  at 
the  serried  crowd  of  peaks,  especially  if  the  mountains 
be  darkly  coloured — covered  with  pines,  for  example  — 
every  peak  and  ridge  is  separated  from  the  mountains 
behind  it  by  a  thin  blue  haze  which  renders  the  relations 
of  the  mountains  as  to  distance  unmistakable.  When 
this  haze  is  regarded  through  the  Nicol  perpendicular 
to  the  sun's  rays,  it  is  in  many  cases  wholly  quenched, 
because  the  light  which  it  emits  in  this  direction  is  wholly 
polarised.  When  this  happens,  aerial  perspective  is 
abolished,  and  mountains  very  differently  distant  appear 
to  rise  in  the  same  vertical  plane.  Close  to  the  Bel 
Alp,  for  instance,  is  the  gorge  of  the  Massa,  and  beyond 
the  gorge  is  a  high  ridge  darkened  by  pines.  This 
ridge  may  be  projected  upon  the  dark  slopes  at  the 
opposite  side  of  the  Rhone  valley,  and  between  both 


128  FEAGMENTS  OF  SCIENCE. 

we  have  the  blue  haze  referred  to,  throwing  the  dis- 
tant mountains  far  away.  But  at  certain  hours  of  the 
day  the  haze  may  be  quenched,  and  then  the  Massa 
ridge  and  the  mountains  beyond  the  Rhone  seem  almost 
equally  distant  from  the  eye.  The  one  appears,  as  it 
were,  a  vertical  continuation  of  the  other.  The  haze 
varies  with  the  temperature  and  humidity  of  the  atmo- 
sphere. At  certain  times  and  places  it  is  almost  as 
blue  as  the  sky  itself;  but  to  see  its  colour,  the  atten- 
tion must  be  withdrawn  from  the  mountains  and  from 
the  trees  which  cover  them.  In  point  of  fact,  the  haze 
is  a  piece  of  more  or  less  perfect  sky ;  it  is  produced 
in  the  same  manner,  and  is  subject  to  the  same 
laws,  as  the  firmament  itself.  We  live  m  the  sky,  not 
under  it. 

These  points  were  further  elucidated  by  the  deport- 
ment of  the  selenite  plate,  with  which  the  readers  of 
the  foregoing  pages  are  so  well  acquainted.  On  some 
of  the  sunny  days  of  August  the  haze  in  the  valley  of 
the  Rhone,  as  looked  at  from  the  Bel  Alp,  was  very  re- 
markable. Towards  evening  the  sky  above  the  moun- 
tains opposite  to  my  place  of  observation  yielded  a 
series  of  the  most  splendidly-coloured  iris-rings  ;  but 
on  lowering  the  selenite  until  it  had  the  darkness  of  the 
pines  at  the  opposite  side  of  the  Rhone  valley,  instead 
of  the  darkness  of  space,  as  a  background,  the  colours 
were  not  much  diminished  in  brilliancy.  I  should 
estimate  the  distance  across  the  valley,  as  the  crow 
flies,  to  the  opposite  mountain,  at  nine  miles ;  so  that  a 
body  of  air  of  this  thickness  can,  under  favourable  cir- 
cumstances, produce  chromatic  effects  of  polarisation 
almost  as  vivid  as  those  produced  by  the  sky  itself. 

Again :  the  light  of  a  landscape,  as  of  most  other 
things,  consists  of  two  parts ;  the  one,  coming  purely 
from  superficial  reflection,  is  always  of  the  same  colour 


AETIFICIAL  SKY.  129 

as  the  light  which  falls  upon  the  landscape ;  the  other 
part  reaches  us  from  a  certain  depth  within  the  objects 
which  compose  the  landscape,  and  it  is  this  portion  of 
the  total  light  which  gives  these  objects  their  distinc- 
tive colours.  The  white  light  of  the  sun  enters  all 
substances  to  a  certain  depth,  and  is  partly  ejected  by 
internal  reflection ;  each  distinct  substance  absorbing 
and  reflecting  the  light,  in  accordance  with  the  laws  of 
its  own  molecular  constitution.  Thus  the  solar  light  is 
sifted  by  the  landscape,  which  appears  in  such  colours 
and  variations  of  colour  as,  after  the  sifting  process, 
reach  the  observer's  eye.  Thus  the  bright  green  of 
grass,  or  the  darker  colour  of  the  pine,  never  comes  to 
us  alone,  but  is  always  mingled  with  an  amount  of  light 
derived  from  superficial  reflection.  A  certain  hard 
brilliancy  is  conferred  upon  the  woods  and  meadows  by 
this  superficially-reflected  light.  Under  certain  cir- 
cumstances, it  may  be  quenched  by  a  Nicol's  prism,  and 
we  then  obtain  the  true  colour  of  the  grass  and  foliage. 
Trees  and  meadows,  thus  regarded,  exhibit  a  richness 
and  softness  of  tint  which  they  never  show  as  long  as 
the  superficial  light  is  permitted  to  mingle  with  the 
true  interior  emission.  The  needles  of  the  pines  show 
this  effect  very  well,  large-leaved  trees  still  better  ; 
while  a  glimmering  field  of  maize  exhibits  the  most 
extraordinary  variations  when  looked  at  through  the 
rotating  Nicol. 

Thoughts  and  questions  like  those  here  referred  to 
took  me,  in  August  1869,  to  the  top  of  the  Aletsch- 
horn.  The  effects  described  in  the  foregoing  para- 
graphs were  for  the  most  part  reproduced  on  the  summit 
of  the  mountain.  I  scanned  the  whole  of  the  sky  with 
my  Nicol.  Both  alone,  and  in  conjunction  with  the 
selenite,  it  pronounced  the  perpendicular  to  the  solar 
beams  to  be  the  direction  of  maximum  polarisation- 


130  FRAGMENTS  OF  SCIENCE. 

But  at  no  portion  of  the  firmament  was  the  polarisation 
complete.  The  artificial  sky  produced  in  the  experi- 
ments recorded  in  the  preceding  pages  could,  in  this 
respect,  be  rendered  far  more  perfect  than  the  natural 
one ;  while  the  gorgeous  '  residual  blue  '  which  makes 
its  appearance  when  the  polarisation  of  the  artificial 
sky  ceases  to  be  perfect,  was  strongly  contrasted  with 
the  lack-lustre  hue  which,  in  the  case  of  the  firmament, 
outlived  the  extinction  of  the  brilliancy.  With  certain 
substances,  however,  artificially  treated,  this  dull  residue 
may  also  be  obtained. 

All  along  the  arc  from  the  Matterhom  to  Mont 
Blanc  the  light  of  the  sky  immediately  above  the 
mountains  was  powerfully  acted  upon  by  the  Nicol. 
In  some  cases  the  variations  of  intensity  were  astonish- 
ing. I  have  already  said  that  a  little  practice  enables 
the  observer  to  shift  the  Nicol  from  one  position  to 
another  so  rapidly  as  to  render  the  alternative  ex- 
tinction and  restoration  of  the  light  immediate. 
When  this  was  done  along  the  arc  to  which  I  have 
referred,  the  alternations  of  light  and  darkness  resembled 
the  play  of  sheet  lightning  behind  the  mountains. 
There  was  an  element  of  awe  connected  with  the  sud- 
denness with  which  the  mighty  masses,  ranged  along 
the  line  referred  to,  changed  their  aspect  and  definition 
under  the  operation  of  the  prifim. 


[In  t!iG  last  edition  of  the  'Fragments  of  Science  '  an  essay 
on  *  Dust  and  Disease '  followed  here  ;  but  as  almost  dU  my 
writings  on  the  '  Germ  Theory  *  are  now  collected  in  a  single 
volume  entitled  '  Essays  on  the  Floating  Matter  of  the  Air,' 
*  Dust  and  Disease  *  no  longer  appears  in  the  *  Fragments.'  In 
its  place  I  venture  to  introduce  a  short  article  written  early 
last  year  fur  an  important  American  magazine.] 


V. 

THE  SKY} 

Invited  to  write  for  the  *  Forum*  an  article  that 
would  have  brought  me  face  to  face  with  '  problems  of 
life  and  mind  '  for  which  I  was  at  the  moment  unpre- 
pared, and  unwilling  to  decline  a  request  so  courteously 
made,  I  offered,  if  the  editor  cared  to  accept  it,  to  send 
him  a  contribution  on  the  subject  here  presented. 

I  mentioned  this  subject,  thinking  that,  in  addition 
to  its  interest  as  a  fragment  of  '  natural  knowledge,'  it 
might  permit  of  a  giance  at  the  workings  of  the 
scientific  mind  when  engaged  on  the  deeper  problems 
which  come  before  it.  In  the  house  of  Science  are 
many  mansions,  occupied  by  tenants  of  diverse  kinds. 
Some  of  them  execute  with  painstaking  fidelity  the 
useful  work  of  observation,  recording  from  day  to  day 
the  aspects  of  Nature,  or  the  indications  of  instruments 
devised  to  reveal  her  ways.  Others  there  are  who  add 
to  this  capacity  for  observation  a  power  over  the 
language  of  experiment,  by  means  of  which  they  put 
questions  to  Nature,  and  receive  from  her  intelligible 
replies.  There  is,  again,  a  third  class  of  minds,  that 
cannot  rest  content  with  observation  and  experiment, 
»  From  •  The  Forum,'  February  1888. 


132  FEAGMENTS  OF  SCIENCE. 

whose  love  of  causal  unity  tempts  them  perpetually 
to  break  through  the  limitations  of  the  senses,  and  to 
seek  beyond  them  the  roots  and  reasons  of  the  pheno- 
mena which  the  observer  and  experimenter  record. 
To  such  spirits — adventurous  and  firm — we  are  indebted 
for  our  deeper  knowledge  of  the  methods  by  which  the 
physical  universe  is  ordered  and  ruled. 

In  his  efforts  to  cross  the  common  bourne  of  the 
known  and  the  unknown,  the  effective  force  of  the  man 
of  science  must  depend,  to  a  great  extent,  upon  his 
acquired  knowledge.  But  knowledge  alone  will  not 
do  ;  a  stored  memory  will  not  suffice ;  inspiration  must 
lend  its  aid.  Scientific  inspiration,  however,  is  usually, 
if  not  always,  the  fruit  of  long  reflection — of  patiently 
'intending  the  mind,'  as  Newton  phrased  it;  and  as 
Copernicus,  Newton,  and  Darwin  practised  it;  until 
outer  darkness  yields  a  glimmer,  which  in  due  time 
opens  out  into  perfect  intellectual  day.  From  some  of 
his  expressions  it  might  be  inferred  that  Newton  scorned 
hypotheses ;  but  he  allowed  them,  nevertheless,  an  open 
avenue  to  his  own  mind.  He  propounded  the  famous 
corpuscular  theory  of  light,  illustrating  it  and  defending 
it  with  a  skill,  power,  and  fascination  which  sub- 
sequently won  for  it  ardent  supporters  among  the  best 
intellects  of  the  world.  This  theory,  moreover,  was 
weighted  with  a  supplementary  hypothesis,  which  as- 
cribed to  the  luminiferous  molecules  '  fits  of  easy 
reflection  and  transmission,'  in  virtue  of  which  they  were 
sometimes  repelled  from  the  surfaces  of  bodies  and 
sometimes  permitted  to  pass  through.  Newton  may 
have  scorned  the  levity  with  which  hypotheses  are 
sometimes  framed ;  but  he  lived  in  an  atmosphere  of 
theory,  which  he,  like  all  profound  scientific  thinkers, 
found  to  be  the  very  breath  of  his  intellectual  life. 

The  theorist  takes  his  conceptions  from  the  world 


THE  SKY.  133 

of  fact,  and  refines  and  alters  them  to  suit  his  needs. 
The  sensation  of  sound  was  known  to  be  produced  by- 
aerial  waves  impinging  on  the  auditory  nerve.  Air  being 
a  thing  that  could  be  felt,  aud  its  vibrations,  by  suitable 
treatment,  made  manifest  to  the  eye,  there  was  here  a 
physical  basis  for  the  '  scientific  imagination '  to  build 
upon.  Both  Hooke  and  Huyghens  built  upon  it  with 
effect.  By  the  illustrious  astronomer  last  named  the 
conception  of  waves  was  definitely  transplanted  from 
its  terrestrial  birthplace  to  a  universal  medium  whose 
undulations  could  only  be  intellectually  discerned. 
Huyghens  did  not  establit^h  the  undulatory  theory,  but 
he  took  the  first  firm  step  towards  establishing  it. 
Laying  this  theory  at  the  root  of  the  phenomena  of 
light,  he  went  a  good  way  towards  showing  that  these 
phenomena  are  the  necessary  outgrowth  of  the  concep- 
tion. 

By  analysis  and  synthesis  Newton  proved  the  white 
light  of  the  sun  to  be  a  skein  of  many  colours.  The 
cause  of  colour  was  a  question  which  immediately  oc- 
cupied his  thoughts ;  and  here,  as  in  other  cases,  he 
freely  resorted  to  hypothesis.  He  saw,  with  his  mind's 
eye,  his  luminiferous  corpuscles  crossing  the  bodily  eye, 
and  imparting  successive  shocks  to  the  retina  behind. 
To  differences  of  '  bigness '  in  the  light-awakening 
molecules  Newton  ascribed  the  different  colour-sensa- 
tions. In  the  undulatory  theory  we  are  also  confronted 
with  the  question  of  colour ;  and  here  again,  to  inform 
and  guide  us,  we  have  the  analogy  of  sound.  Aerial 
waves  of  different  lengths,  or  periods,  produce  notes  of 
different  pitch ;  and  to  differences  of  wave-length  in 
that  mysterious  medium,  the  all- pervading  ether,  dif- 
ferences of  colour  are  ascribed.  Hooke  had  already 
discoursed  of  '  a  very  quick  motion  that  causes  light,  as 
well  as  a  more  robust  that  causes  heat.'     Newton  had, 


r.^4  FEAGMENTS  OF  SCIENCE. 

ascribed  the  sensation  of  red  to  the  shock  of  his  grossest, 
and  that  of  violet  to  the  shock  of  his  finest  luminiferous 
projectiles.  Defining  the  one,  and  displacing  the  other 
of  these  notions,  the  wave-theory  affirms  red  to  be  pro- 
duced by  the  largest,  and  violet  by  the  smallest  waves 
of  the  visible  spectrum.  The  theory  of  undulation  had 
to  encounter  that  fierce  struggle  for  existence  which 
all  great  changes  of  doctrine,  scientific  or  otherwise, 
have  had  to  endure.  Mighty  intellects,  following  the 
mightiest  of  them  all,  were  arrayed  against  it.  But  the 
more  it  was  discussed  the  more  it  grow  in  strength  and 
favour,  until  it  finally  supplanted  its  formidable  rival. 
No  competent  scientific  man  at  the  present  day  accepts 
the  theory  of  emission,  or  refuses  to  accept  the  theory 
of  undulation. 

Boyle  and  Hooke  had  been  fruitful  experimenters  on 
those  beautiful  iridescences  known  as  the  'colours  of 
thin  plates.'  The  rich  hues  of  the  thin-blown  soap- 
bubble,  of  oil  floating  on  water,  and  of  the  thin  layer  of 
oxide  on  molten  lead,  are  familiar  illustrations  of  these 
iris  colours.  Hooke  showed  that  all  transparent  films,  if 
only  thin  enough,  displayed  such  colours ;  and  he  proved 
that  the  particular  colour  displayed  depended  upon  the 
thickness  of  the  film.  Passing  from  solid  and  liquid 
films  to  films  of  air,  he  says :  '  Take  two  small  pieces  of 
ground  and  polished  looking-glass  plate,  each  about  the 
bigness  of  a  shilling ;  take  these  two  dry,  and  with  your 
forefingers  and  thumbs  press  them  very  hard  and  close 
together,  and  you  shall  find  that  when  they  approach 
each  other  very  near,  there  will  appear  several  irises  or 
coloured  lines.'  Newton,  bent  on  knowing  the  exact 
relation  between  the  thickness  of  the  film  and  the  colour 
it  produced,  varied  Hooke's  experiment.  Taking  two 
pieces  of  glass,  the  one  plane  and  the  other  very  slightly 
curved,  and  pressing  both  together,  he  obtained  a  film 


THE  SKY.  135 

of  air  of  gradually  increasing  thickness  from  the  place 
of  contact  outwards.  As  he  expected,  he  found  the  place 
of  contact  surrounded  by  a  series  of  coloured  circles, 
still  known  all  over  the  world  as  'Newton's  rings.'  The 
colours  of  his  first  circle,  which  immediately  surrounded 
a  black  central  spot,  Newton  called  '  colours  of  the  first 
order  ; '  the  colours  of  the  second  circle,  '  colours  of  the 
second  order,'  and  so  on.  With  unrivalled  penetration 
and  apparent  success,  he  applied  his  theory  of  '  fits' 
to  the  explanation  of  the  *  rings.'  Here,  however,  the 
only  immortal  parts  of  his  labours  are  his  facts  and 
measurements ;  his  theory  has  disappeared.  It  was  re- 
served for  the  illustrious  Thomas  Young,  a  man  of  in- 
tellectual calibre  resembling  that  of  Newton  himself,  to 
prove  that  the  rings  were  produced  by  the  mutual  action 
— in  technical  plirase,  'interference' — of  the  light- 
waves reflected  at  the  two  surfaces  of  the  film  of  air 
inclosed  between  the  plane  and  convex  glasses.  The 
colours  of  thin  plates  were  'residual  colours' — survivals 
of  the  white  light  after  the  ravages  of  interference. 
Young  soon  translated  the  theory  of  'fits'  into  that  of 
'  waves ; '  the  measurements  pertaining  to  the  former 
being  so  accurate  as  to  render  them  immediately 
available  for  the  purposes  of  the  latter. 

It  is  here  that  Newton's  researches  and  opinions 
touch  the  subject  of  this  article.  The  colour  nearest  to 
the  black  spot,  in  the  experiment  above  described,  was 
a  faint  blue — '  blue  of  the  first  order  ' — corresponding 
to  the  film  of  air  when  thinnest.  If  a  solid  or  liquid 
film,  of  the  thickness  requisite  to  produce  this  colour, 
were  broken  into  bits  and  scattered  in  the  air,  Newton 
inferred  that  the  tiny  fragments  would  display  the  blue 
colour.  Tantamount  to  this,  he  considered,  was  the  ac- 
tion of  minute  water-particles  in  the  incipient  stage  of 
their  condensation  from  aqueous  vapour.    Such  particles 

10 


136  FKAGMENTS  OF  SCIBNCE. 

suspended  in  our  atmosphere  ought,  he  supposed,  to 
generate  the  serenest  skies.  Newton  does  not  appear 
to  have  bestowed  much  thought  upon  this  subject ;  for 
to  produce  the  particular  blue  which  he  regarded  as 
sky-blue,  thin  plates  with  parallel  surfaces  would  be 
required.  The  notion  that  cloud-particles  are  hollow 
spheres,  or  vesicles,  is  prevalent  on  the  Continent,  but  it 
never  made  any  way  among  the  scientific  men  of  Eng- 
land. De  Saussure  thought  that  he  had  actually  seen 
the  cloud-vesicles,  and  Faraday,  as  I  learned  from 
himself,  believed  that  he  had  once  confirmed  the  observa- 
tion of  the  illustrious  Alpine  traveller.  During  my  long 
acquaintance  with  the  atmosphere  of  the  Alps  I  have 
often  sought  for  these  aqueous  bladders,  but  have  never 
been  able  to  find  them.  Clausius  once  published  a 
profound  essay  on  the  colours  of  the  sky.  The  assump- 
tion of  small  water  drops,  he  proved,  would  lead  to 
optical  consequences  entirely  at  variance  with  facts. 
For  a  time,  therefore,  he  closed  with  the  idea  of  vesicles, 
and  endeavoured  to  deduce  from  them  the  blue  of  the 
firmament  and  the  morning  and  evening  red. 

It  is  not,  however,  necessary  to  invoke  the  blue  of 
the  first  order  to  explain  the  colour  of  the  sky  ;  nor  is 
it  necessary  to  impose  upon  condensing  vapour  the  diffi- 
cult, if  not  impossible,  task  of  forming  bladders,  when 
it  passes  into  the  liquid  condition.  Let  us  examine 
the  subject.  Eau-de-Cologne  is  prepared  by  dissolving 
aromatic  gums  or  resins  in  alcohol.  Dropped  into 
water,  the  scented  liquid  immediately  produces  a  white 
cloudiness,  due  to  the  precipitation  of  the  substances 
previously  held  in  solution.  The  solid  particles  are, 
however,  comparatively  gross ;  but  by  diminishing  the 
quantity  of  the  dissolved  gum,  the  precipitate  may 
be  made  to  consist  of  extremely  minute  particles. 
Briicke,  for  example,  dissolved  gum-mastic,  in  certain 


THE  SKY.  137 

proportions,  in  alcohol,  and  carefully  dropping  his 
solution  into  a  beaker  of  water,  kept  briskly  stirred,  he 
was  able  to  reduce  the  precipitate  to  an  extremely  fine 
state  of  division.  The  particles  of  mastic  can  by  no 
means  be  imagined  as  forming  bladders.  Still,  against 
a  dark  ground — black  velvet,  for  example — the  water 
that  contains  them  shows  a  distinctly  blue  colour.  The 
bluish  colour  of  many  liquids  is  produced  in  a  similar 
manner.  Thin  milk  is  an  example.  Blue  eyes  are 
also  said  to  be  simply  turbid  media.  The  rocks  over 
which  glaciers  pass  are  finely  ground  and  pulverised  by 
the  ice,  or  the  stony  emery  imbedded  in  it ;  and  the 
river  which  issues  from  the  snout  of  every  glacier  is 
laden  with  suspended  matter.  When  such  glacier  water 
is  placed  in  a  tall  glass  jar,  and  the  heavier  particles 
are  permitted  to  subside,  the  liquid  column,  when 
viewed  against  a  dark  background,  has  a  decidedly 
bluish  tinge.  The  exceptional  blueness  of  the  Lake  of 
Geneva,  which  is  fed  with  glacier  water,  may  be  due, 
in  part,  to  particles  small  enough  to  remain  suspended 
long  after  their  larger  and  heavier  companions  have 
sunk  to  the  bottom  of  the  lake. 

We  need  not,  however,  resort  to  water  for  the  pro- 
duction of  the  colour.  We  can  liberate,  in  air,  particles 
of  a  size  capable  of  producing  a  blue  as  deep  and  pure 
as  the  azure  of  the  firmament.  In  fact,  artificial  skies 
may  be  thus  generated,  which  prove  their  brotherhood 
with  the  natural  sky  by  exhibiting  all  its  phenomena. 
There  are  certain  chemical  compounds — aggregates  of 
molecules — the  constituent  atoms  of  which  are  readily 
shaken  asunder  by  the  impact  of  special  waves  of  light. 
Probably,  if  not  certainly,  the  atoms  and  the  waves 
are  so  related  to  each  other,  as  regards  vibrating  period, 
that  the  wave-motion  can  accumulate  until  it  becomes 
disruptive.     A  great  number  of  substances  might  be 


138  FRAGMENTS  OF  SCIENCE. 

mentioned  whose  vapours,  when  mixed  with  air  and  sub- 
jected to  the  action  of  a  solar  or  an  electric  beam,  are 
thus  decomposed,  the  products  of  decomposition  hanging 
as  liquid  or  solid  particles  in  the  beam  which  generates 
them.  And  here  I  must  appeal  to  the  inner  vision  already 
spoken  of.  Eemembering  the  different  sizes  of  the  waves 
of  light,  it  is  not  difficult  to  see  that  our  minute  par- 
ticles are  larger  with  respect  to  some  waves  than  to 
others.  In  the  case  of  water,  for  example,  a  pebble  will 
intercept  and  reflect  a  larger  fractional  part  of  a  ripple 
than  of  a  larger  wave.  We  have  now  to  imagine  light- 
undulations  of  different  dimensions,  but  all  exceedingly 
minute,  passing  through  air  laden  with  extremely  small 
particles.  It  is  plain  that  such  particles,  though  scat- 
tering portions  of  all  the  waves,  will  exert  their  most 
conspicuous  action  upon  the  smallest  ones ;  and  that 
the  colour-sensation  answering  to  the  smallest  waves — 
in  other  words,  the  colour  blue — will  be  predominant 
in  the  scattered  light.  This  harmonises  perfectly  with 
what  we  observe  in  the  firmament.  The  sky  is  blue, 
but  the  blue  is  not  pure.  On  looking  at  the  sky 
through  a  spectroscope,  we  observe  all  the  colours  of  the 
spectrum;  blue  is  merely  the  predominant  colour. 
By  means  of  our  artificial  skies  we  can  take,  as  it  were, 
the  firmament  in  our  hands  and  examine  it  at  our 
leisure, .  Like  the  natural  sky,  the  artificial  one  shows 
all  the  colours  of  the  spectrum,  but  blue  in  excess. 
Mixing  very  small  quantities  of  vapour  with  air,  and 
bringing  the  decomposing  luminous  beam  into  action, 
we  produce  particles  too  small  to  shed  any  sensible 
light,  but  which  may,  and  doubtless  do,  exert  an  action 
on  the  ultra-violet  waves  of  the  spectrum.  We  can 
watch  these  particles,  or  rather  the  space  they  occupy, 
till  they  grow  to  a  size  able  to  yield  the  firmamental 
cizure.     As  the  particles  grow  larger  under  the  continued 


THE  SKY.  139 

action  of  the  light,  the  azure  becomes  less  deep ;  while 
later  on  a  milkiness,  such  as  we  often  observe  in  nature, 
takes  the  place  of  the  purer  blue.  Finally  the  particles 
become  large  enough  to  reflect  all  the  light-  waves,  and 
then  the  suspended  '  actinic  cloud  '  diffuses  white  light. 
It  must  occur  to  the  reader  that  even  in  the  absence 
of  definite  clouds  there  are  considerable  variations  in 
the  hue  of  the  firmament.  Everybody  knows,  moreover, 
that  as  the  sky  bends  towards  the  horizon,  the  purer 
blue  is  impaired.  To  measure  the  intensity  of  the  colour 
De  Saussure  invented  a  cyanometer,  and  Humboldt  has 
given  us  a  mathematical  formula  to  express  the  diminu- 
tion of  the  blue,  in  arcs  drawn  east  and  west  from  the 
zenith  downwards.  This  diminution  is  a  natural  con* 
sequence  of  the  predominance  of  coarser  particles  in  the 
lower  regions  of  the  atmosphere.  Were  the  particles 
which  produce  the  purer  celestial  vault  all  swept  away, 
we  should,  unless  helped  by  what  has  been  called 
*  cosmic  dust,'  look  into  the  blackness  of  celestial  space. 
.  And  were  the  whole  atmosphere  abolished  along  with  its 
suspended  matter,  we  should  have  the  *  blackness ' 
spangled  with  steady  stars;  for  the  twinkling  of  the 
stars  is  caused  by  our  atmosphere.  Now,  the  higher  we 
ascend,  the  more  do  we  leave  behind  us  the  particles 
which  scatter  the  light;  the  nearer,  in  fact,  do  we  ap- 
proach to  that  vision  of  celestial  space  mentioned  a 
moment  ago.  Viewed,  therefore,  from  the  loftiest 
Alpine  summits,  the  firmamental  blue  is  darker  than  it 
is  ever  observed  to  be  from  the  plains. 

It  is  thus  shown  that  by  the  scattering  action  of 
minute  particles  the  blue  of  the  sky  can  be  produced  ; 
but  there  is  yet  more  to  be  said  upon  the  subject.  Let 
the  natural  sky  be  looked  at  on  a  fine  day  through  a 
piece  of  transparent  Iceland  spar  cut  into  the  ibim 
known  as  a  Nicol  prism.     It  may  be  well  to  begin  by 


140  FRAGMENTS  OF  SCIENCE. 

looking  through  the  prism  at  a  snow  slope,  or  a  white 
wall.  Turning  the  prism  round  its  axis,  the  light  com- 
ing from  these  objects  does  not  undergo  any  sensible 
change.  But  when  the  prism  is  directed  towards  the 
sky  the  great  probability  is  that,  on  turning  it,  variations 
in  the  amount  of  light  reaching  the  eye  will  be  observed. 
Testing  various  portions  of  the  sky  with  due  diligence, 
we  at  length  discover  one  particular  direction  where  the 
difference  of  illumination  becomes  a  maximum.  Here 
the  Nicol,  in  one  position,  seems  to  offer  no  impediment 
to  the  passage  of  the  sky  light ;  while,  when  turned 
through  an  arc  of  ninety  degrees  from  this  position,  the 
light  is  almost  entirely  quenched.  We  soon  discern  that 
the  particular  line  of  vision  in  which  this  maximum 
difference  is  observed  is  perpendicular  to  the  direction 
of  the  solar  rays.  The  Nicol  acts  thus  upon  sky  light 
because  that  light  is  polarised,  while  the  light  from  the 
white  wall  or  the  white  snow,  being  unpolarised,  is  not 
affected  by  the  rotation  of  the  prism. 

In  the  case  of  our  manufactured  sky  not  only  is  the 
azure  of  the  firmament  reproduced,  but  these  phenomena 
of  polarisation  are  observed  even  more  perfectly  than 
in  the  natural  sky.  When  the  air-space  from  which 
our  best  artificial  azure  is  emitted  is  examined  with  the 
Nicol  prism,  the  blue  light  is  found  to  be  completely 
polarised  at  right  angles  to  the  illuminating  beam. 
The  artifical  sky  may,  in  fact,  be  employed  as  a  second 
Nicol,  between  which  and  a  prism  held  in  the  hand 
many  of  the  beautiful  chromatic  phenomena  observed 
in  an. ordinary  polariscope  may  be  reproduced. 

Let  us  now  complete  our  thesis  by  following  the 
larger  light-waves,  which  have  been  able  to  pass  among 
the  aerial  particles  with  comparatively  little  fractional 
loss.  Without  going  beyond  inferential  considerations, 
we   can   state   what  must  occur.     The  action  of  the 


THE  SKY.  141 

particles  upon  the  solar  light  increases  with  the  atmo- 
spheric distances  traversed  by  the  sun's  rays.  The 
lower  the  sun,  therefore,  the  greater  the  action.  The 
shorter  waves  of  the  spectrum  being  more  and  more 
withdrawn,  the  tendency  is  to  give  the  longer  waves 
an  enchanced  predominance  in  the  transmitted  light. 
The  tendency,  in  other  words,  of  this  light,  as  the  rays 
traverse  ever-increasing  distances,  is  more  and  more 
towards  red.  This,  I  say,  might  be  stated  as  an  in- 
ference, but  it  is  borne  out  in  the  most  impressive 
manner  by  facts.  When  the  Alpine  sun  is  setting,  or, 
better  still,  some  time  after  he  has  set,  leaving  the  limbs 
and  shoulders  of  the  mountains  in  shadow,  while  their 
snowy  crests  are  bathed  by  the  retreating  light,  the 
snow  glows  with  a  beauty  and  solemnity  hardly  equalled 
by  any  other  natural  phenomenon.  So,  also,  when  first 
illumined  by  the  rays  of  the  unrisen  sun,  the  moun- 
tain heads,  under  favourable  atmospheric  conditions, 
shine  like  rubies.  And  all  this  splendour  is  evoked  by 
the  simple  mechanism  of  minute  particles,  themselves 
without  colour,  suspended  in  the  air.  Those  who 
referred  the  extraordinary  succession  of  atmospherie 
glows,  witnessed  some  years  ago,  to  a  vast  and  violent 
discharge  of  volcanic  ashes,  were  dealing  with  '  a  true 
cause.'  The  fine  floating  residue  of  such  ashes  would, 
undoubtedly,  be  able  to  produce  the  effects  ascribed  to 
it.  Still,  the  mechanism  necessary  to  produce  the 
morning  and  the  evening  red,  though  of  variable  effi- 
ciency, is  always  present  in  the  atmosphere.  I  have 
seen  displays,  equal  in  magnificence  to  the  finest  of  those 
above  referred  to,  when  there  was  no  special  volcanic 
outburst  to  which  they  could  be  referred.  It  was  the 
long-continued  repetition  of  the  glows  which  rendered 
the  volcanic  theory  highly  probable. 


142  FEAGMENTS  OF  SCIENCE. 


VL 


VOYAGE  TO  ALGERIA    TO   OBSERVE  THE 
ECLIPSE. 

1870. 

THE  opening  of  the  Eclipse  Expedition  was  not  pro- 
pitious. Portsmouth,  on  Monday,  December  5. 
1870,  was  swathed  by  fog,  which  was  intensified  by  smoke, 
and  traversed  by  a  drizzle  of  fine  rain.  At  six  p.m.  I 
was  on  board  the  '  Urgent.'  On  Tuesday  morning  the 
weather  was  too  thick  to  permit  of  the  ship's  being 
swung  and  her  compasses  calibrated.  The  Admiral  of 
the  port,  a  man  of  very  noble  presence,  came  on  board. 
Under  his  stimulus  the  energy  which  the  weather  had 
damped  appeared  to  become  more  active,  and  soon  after 
his  departure  we  steamed  down  to  Spithead.  Here  the 
fog  had  so  far  lightened  as  to  enable  the  officers  to  swing 
the  ship. 

At  three  P.M.  on  Tuesday,  December  6,  we  got  away, 
gliding  successively  past  Whitecliff  Bay,  Bembridge, 
Sandown,  Shanklin,  Ventnor,  and  St.  Catherine's  Light- 
house. On  Wednesday  morning  we  sighted  the  Isle  of 
Ushant,  on  the  French  side  of  the  Channel.  The 
northern  end  of  the  island  has  been  fretted  by  the 
waves  into  detached  tower-like  masses  of  rock  of  very 
remarkable  appearance.  In  the  Channel  the  sea  was 
green,  and  opposite  Ushant  it  was  a  brighter  green.  On 
Wednesday  evening  we  committed  ourselves  to  the  Bay 
of  Biscay.     The  roll  of  the  Atlantic  was  full,  but  not 


VOYAGE  TO  ALGERIA.  143 

violent.  There  had  been  scarcely  a  gleam  of  sunshine 
throughout  the  day,  but  the  cloud-forms  were  fine,  and 
their  apparent  solidity  impressive.  On  Thursday  morn- 
ing the  green  of  the  sea  was  displaced  by  a  deep  indigo 
blue.  The  whole  of  Thursday  we  steamed  across  the 
bay.  We  had  little  blue  sky,  but  the  clouds  were  again 
grand  and  varied — cirrus,  stratus,  cumulus,  and  nimbus, 
we  had  them  all.  Dusky  hair-like  trails  were  some- 
times dropped  from  the  distant  clouds  to  the  sea. 
These  were  falling  showers,  and  they  sometimes  occu- 
pied the  whole  horizon,  while  we  steamed  across  the 
rainless  circle  which  was  thus  surrounded.  Sometimes 
we  plunged  into  the  rain,  and  once  or  twice,  by  slightly 
changing  our  course,  avoided  a  heavy  shower.  From 
time  to  time  perfect  rainbows  spanned  the  heavens  from 
side  to  side.  At  times  a  bow  would  appear  in  frag- 
ments, showing  the  keystone  of  the  arch  midway  in 
air,  and  its  two  buttresses  on  the  horizon.  In  all  cases 
the  light  of  the  bbw  could  be  quenched  by  a  Nicol's 
prism,  with  its  long  diagonal  tangent  to  the  arc. 
Sometimes  gleaming  patches  of  the  firmament  were 
seen  amid  the  clouds.  When  viewed  in  the  proper 
direction,  the  gleam  could  be  quenched  by  a  Nicol's 
prism,  a  dark  aperture  being  thus  opened  into  stellar 
space. 

At  sunset  on  Thursday  the  denser  clouds  were 
fiercely  fringed,  while  through  the  lighter  ones  seemed 
to  issue  the  glow  of  a  conflagration.  On  Friday  morn- 
ing we  sighted  Cape  Finisterre  —  the  extreme  end  of 
the  arc  which  sweeps  from  Ushant  round  the  Bay  of 
Biscay.  Calm  spaces  of  blue,  in  which  floated  quietly 
scraps  of  cumuli,  were  behind  us,  but  in  front  of  us  was 
a  horizon  of  portentous  darkness.  It  continued  thus 
threatening  throughout  the  day.  Towards  evening  the 
wind  strengthened  to  a  gale,  and  at  dinner  it  was  diffi 


144  FKAGMENTS  OF  SCIENCE. 

cult  to  preserve  the  plates  and  dishes  from  destructioD. 
Our  thinned  company  hinted  that  the  rolling  had  other 
consequences.  It  was  very  wild  when  we  went  to  bed. 
I  slumbered  and  slept,  but  after  some  time  was  rendered 
anxiously  conscious  that  my  body  had  become  a  kind 
of  projectile,  with  the  ship's  side  for  a  target.  I  gripped 
the  edge  of  my  berth  to  save  myself  from  being  thrown 
out.  Outside,  I  could  hear  somebody  say  that  he  had 
been  thrown  from  his  berth,  and  sent  spinning  to  the 
other  side  of  the  saloon.  The  screw  laboured  violently 
amid  the  lurching  ;  it  incessantly  quitted  the  water,  and, 
twirling  in  the  air,  rattled  against  its  bearings,  causing 
the  ship  to  shudder  from  stem  to  stern.  At  times  the 
waves  struck  us,  not  with  the  soft  impact  which  might 
be  expected  from  a  liquid,  but  with  the  sudden  solid 
shock  of  battering-rams.  '  No  man  knows  the  force  of 
water,'  said  one  of  the  officers, '  until  he  has  experienced 
a  storm  at  sea.'  These  blows  followed  each  other  at 
quicker  intervals,  the  screw  rattling  after  each  of  them, 
until,  finally,  the  delivery  of  a  heavier  stroke  than 
ordinary  seemed  to  reduce  the  saloon  to  chaos.  Fm-ni- 
ture  crashed,  glasses  rang,  and  alarmed  enquiries 
immediately  followed.  Amid  the  noises  I  heard  one 
note  of  forced  laughter  ;  it  sounded  very  ghastly.  Men 
tramped  through  the  saloon,  and  busy  voices  were  heard 
aft,  as  if  something  there  had  gone  wrong. 

I  rose,  and  not  without  difficulty  got  into  my 
clothe?  In  the  after-cabin,  under  the  superintendence 
of  the  able  and  energetic  navigating  lieutenant,  Mr, 
Brown,  a  group  of  blue-jackets  were  working  at  the 
tiller-ropes.  These  had  become  loose,  and  the  helm 
refused  to  answer  the  wheel.  High  moral  lessons 
might  be  gained  on  shipboard,  by  observing  what  stead- 
fast adherence  to  an  object  can  accomplish,  and  what 
large  effects  are  heaped  up  by  the  addition  of  infinitesi- 


VOYAaE  TO  ALGEEIA.  145 

mals.  The  tiller-rope,  as  the  blue-jackets  strained  in 
concert,  seemed  hardly  to  move ;  still  it  did  move  a 
little,  until  finally,  by  timing  the  pull  to  the  lurching 
of  the  ship,  the  mastery  of  the  rudder  was  obtained.  I 
liad  previously  gone  on  deck.  Bound  the  saloon-door 
were  a  few  members  of  the  eclipse  party,  who  seemed 
in  no  mood  for  scientific  ooservation.  Nor  did  I ;  but 
I  wished  to  see  the  storm.  I  climbed  the  steps  to  the 
poop,  exchanged  a  word  with  Captain  Toynbee,  the 
only  member  of  the  party  to  be  seen  on  the  poop,  and 
by  his  direction  made  towards  a  cleat  not  far  from  the 
wheel.'  Round  it  I  coiled  my  arms.  With  the  excep- 
tion of  the  men  at  the  wheel,  who  stood  as  silent  as 
corpses,  I  was  alone. 

I  had  seen  grandeur  elsewhere,  but  this  was  a  new 
form  of  grandeur  to  me.  The  'Urgent'  is  long  and 
narrow,  and  during  our  expedition  she  lacked  the  steady- 
ing influence  of  sufficient  ballast.  She  was  for  a  time 
practically  rudderless,  and  lay  in  the  trough  of  the  sea. 
I  could  see  the  long  ridges,  with  some  hundreds  of  feet 
between  their  crests,  rolling  upon  the  ship  perfectly 
parallel  to  her  sides.  As  they  approached,  they  so  grew 
upon  the  eye  as  to  render  the  expression  'mountains 
high'  intelligible.  At  all  events,  there  was  no  mistaking 
their  mechanical  might,  as  they  took  the  ship  upon  their 
shoulders,  and  swung  her  like  a  pendulum.  The  deck 
sloped  sometimes  at  an  angle  which  I  estimated  at  over 
forty-five  degrees ;  wanting  my  previous  Alpine  practice, 
I  should  have  felt  less  confidence  in  my  grip  of  the 
cleat.  Here  and  there  the  long  rollers  were  tossed  by 
interference  into  heaps  of  greater  height.  The  wind 
caught  their  crests,  and  scattered  them  over  the  sea,  the 
whole  surface  of  which  was  seething  white.     The  aspect 

*  The  cleat  is  a  T-shaped  mass  of  metal  employed  for  the 
fefitening  of  ropes. 


146  FRAGMENTS  OF  SCIENCE. 

of  the  clouds  was  a  fit  accompaniment  to  the  fury  of  the 
ocean.  The  moon  was  almost  full—  at  times  concealed, 
at  times  revealed,  as  the  scud  flew  wildly  over  it.  These 
things  appealed  to  the  eye,  while  the  ear  was  filled  by 
the  groaning  of  the  screw  and  the  whistle  and  boom  of 
the  storm. 

Nor  was  the  outward  agitation  the  only  object  of 
interest  to  me.  I  was  at  once  subject  and  object  to 
myself,  and  watched  with  intense  interest  the  workings 
of  my  own  mind.  The  'Urgent'  is  an  elderly  ship. 
She  had  been  built,  I  was  told,  by  a  contracting  firm 
for  some  foreign  Government,  and  had  been  diverted 
from  her  first  purpose  when  converted  into  a  troop-ship. 
She  had  been  for  some  time  out  of  work,  and  I  had 
heard  that  one  of  her  boilers,  at  least,  needed  repair. 
Our  scanty  but  excellent  crew,  moreover,  did  not  belong 
to  the  'Urgent,'  but  had  been  gathered  from  other 
ships.  Our  three  lieutenants  were  also  volunteers.  All 
this  passed  swiftly  through  my  mind  as  the  steamer 
shook  under  the  blows  of  the  waves,  and  I  thought  that 
probably  no  one  on  board  could  say  how  much  of  this 
thumping  and  straining  the  'Urgent'  would  be  able  to 
bear.  This  uncertainty  caused  me  to  look  steadily  at 
the  worst,  and  I  tried  to  strengthen  myself  in  the  face 
of  it. 

But  at  length  the  helm  laid  hold  of  the  water,  and 
the  ship  was  got  gradually  round  to  face  the  waves. 
The  rolliug  diminished,  a  certain  amount  of  pitching 
taking  its  place.  Our  speed  had  fallen  from  eleven 
knots  to  two.  I  went  again  to  bed.  After  a  space  of 
calm,  when  we  seemed  crossing  the  vortex  of  a  storm, 
heavy  tossing  recommenced.  I  was  afraid  to  allow 
myself  to  fall  asleep,  as  my  berth  was  high,  and  to  be 
pitched  out  of  it  might  be  attended  with  bruises,  if  not 
with  fractures.     From  Friday  at  noon  to  Saturday  a* 


VOYAGE  TO  AXGEEIA.  147 

noon  we  accomplished  sixty-six  miles,  or  an  average  of 
less  than  three  miles  an  hour.  I  overheard  the  sailors 
talking  about  this  storm.  The  '  Urgent,'  according  to 
those  that  knew  her,  had  never  previously  experienced 
anything  like  it.^ 

All  through  Saturday  the  wind,  though  somewhat 
sobered,  blew  dead  against  us.  The  atmospheric  effects 
were  exceedingly  fine.  The  cumuli  resembled  moun- 
tains in  shape,  and  their  peaked  summits  shone  as  white 
as  Alpine  snows.  At  one  place  this  resemblance  was 
greatly  strengthened  by  a  vast  area  of  cloud,  uniformly 
illuminated,  and  lying  like  a  nev6  below  the  peaks. 
From  it  fell  a  kind  of  cloud-river  strikingly  like  a 
glacier.  The  horizon  at  sunset  was  remarkable — spaces 
of  brilliant  green  between  clouds  of  fiery  red.  Kain- 
bows  had  been  frequent  throughout  the  day,  and  at 
night  a  perfectly  continuous  lunar  bow  spanned  the 
heavens  from  side  to  side.  Its  colours  were  feeble ; 
but,  contrasted  with  the  black  ground  against  which  it 
rested,  its  luminousness  was  extraordinary. 

Sunday  morning  found  us  opposite  to  Lisbon,  and 
at  midnight  we  rounded  Cape  St.  Vincent,  where  the 
lurching  seemed  disposed  to  recommence.  Through 
the  kindness  of  Lieutenant  Walton,  a  cot  had  been 
slung  for  me.  It  hung  between  a  tiller-wheel  and  a 
flue,  and  at  one  a.m.  I  was  roused  by  the  banging  of  the 
cot  against  its  boundaries.  But  the  wind  was  now 
behind  us,  and  we  went  along  at  a  speed  of  eleven  knots. 
We  felt  certain  of  reaching  Cadiz  by  three.  But  a  new 
lighthouse  came  in  sight,  which  some  affirmed  to  be 
Cadiz  Lighthouse,  while  the  surrounding  houses  were  de- 
clared to  be  those  of  Cadiz  itself.     Out  of  deference  to 

'  There  is,  it  will  be  seen,  a  fair  agreement  between  these  im- 
pressions and  those  so  vigorously  described  by  a  scientific  corro- 
spocdent  of  the  *  Times.' 


148  FKAGMENTS  OF  SCIENCE. 

these  statements,  the  navigating  lieutenant  changed  hia 
course,  and  steered  for  the  place.  A  pilot  came  on  board, 
and  he  informed  us  that  we  were  before  the  mouth 
of  the  Guadalquivir,  and  that  the  lighthouse  was  that 
of  Cipiona.   Cadiz  was  still  some  eighteen  miles  distant. 

We  steered  towards  the  city,  hoping  to  get  into  the 
harbour  before  dark.  But  the  pilot  who  would  have 
guided  us  had  been  snapped  up  by  another  vessel,  and 
we  did  not  get  in.  We  beat  about  during  the  night, 
and  in  the  morning  found  ourselves  about  fifteen 
miles  from  Cadiz.  The  sun  rose  behind  the  city, 
and  we  steered  straight  into  the  light.  The  three- 
towered  cathedral  stood  in  the  midst,  round  which 
swarmed  apparently  a  multitude  of  chimney-stacks.  A 
nearer  approach  showed  the  chimneys  to  be  small  turrets. 
A  pilot  was  taken  on  board ;  for  there  is  a  dangerous 
shoal  in  the  harbour.  The  appearance  of  the  town  as 
the  sun  shone  upon  its  white  and  lofty  walls  was  singu- 
larly beautiful.  We  cast  anchor ;  some  officials  arrived 
and  demanded  a  clean  bill  of  health.  We  had  none. 
They  would  have  nothing  to  do  with  us ;  so  the  yellow 
quarantine  flag  was  hoisted,  and  we  waited  for  permis- 
sion to  land  the  Cadiz  party.  After  some  hours'  delay 
the  English  consul  and  vice-consul  came  on  board,  and 
with  them  a  Spanish  officer  ablaze  with  gold  lace  and 
decorations.  Under  slight  pressure  the  requisite  permis- 
sion had  been  granted.  We  landed  our  party,  and  in 
the  afternoon  weighed  anchor.  Thanks  to  the  kindness 
of  owe  excellent  paymaster,  I  was  here  transferred  to  a 
more  roomy  berth. 

Cadiz  soon  sank  beneath  the  sea,  and  we  sighted  in 
succession  Cape  Trafalgar,  Tarifa,  and  the  revolving 
light  of  Ceuta.  The  water  was  very  calm,  and  the  moon 
rose  in  a  quiet  heaven.  She  swung  with  her  convex 
surface  downwards,  the  common  boundary  between  light 


VOYAGE  TO  ALGEEIA.  149 

and  shadow  being  almost  horizontal.  A  pillar  of  reflected 
light  shimmered  up  to  us  from  the  slightly  rippled  sea. 
I  had  previously  noticed  the  phosphorescence  of  the 
water,  but  to  night  it  was  stronger  than  usual,  especially 
among  the  foam  at  the  bows.  A  bucket  let  down  into 
the  sea  brought  up  a  number  of  the  little  sparkling 
organisms  which  caused  the  phosphorescence.  I  caught 
some  of  them  in  my  hand.  And  here  an  appearance 
was  observed  which  was  new  to  most  of  us,  and  strik- 
ingly beautiful  to  all.  Standing  at  the  bow  and  looking 
forwards,  at  a  distance  of  forty  or  fifty  yards  from  the 
ship,  a  number  of  luminous  streamers  were  seen  rushing 
towards  us.  On  nearing  the  vessel  they  rapidly  turned, 
like  a  comet  round  its  perihelion,  placed  themselves 
side  by  side,  and,  in  parallel  trails  of  light,  kept  up  with 
the  ship.  One  of  them  placed  itself  right  in  front  of 
the  bow  as  a  pioneer.  These  comets  of  the  sea  were 
joined  at  intervals  by  others.  Sometimes  as  many  as 
six  at  a  time  would  rush  at  us,  bend  with  extraordinary 
rapidity  round  a  sharp  curve,  and  afterwards  keep  us 
company.  T  leaned  over  the  bow,  and  scanned  the 
streamers  closely.  The  frontal  portion  of  each  of  them 
revealed  the  outline  of  a  porpoise.  The  rush  of  the 
creatures  through  the  water  had  started  the  phosphor- 
escence,  every  spark  of  which  was  converted  by  the 
motion  of  the  retina  into  a  line  of  light.  Each  porpoise 
was  thus  wrapped  in  a  limiinous  sheath.  The  phospho- 
rescence did  not  cease  at  the  creature's  tail,  but  was 
carried  many  porpoise-lengths  behind  it. 

To  our  right  we  had  the  African  hills,  illuminated 
by  the  moon.  Gibraltar  Eock  at  length  became  visible, 
but  the  town  remained  long  hidden  by  a  belt  of  haze, 
through  which  at  length  the  brighter  lamps  struggled. 
It  was  like  the  gradual  resolution  of  a  nebula  into 
stars.     As  the  intervening  depth  became  gradually  less. 


150  i^EAGMENTS  OF  SCIENCE. 

the  mist  vanished  more  and  more,  and  finally  all  the 
lamps  shone  through  it  They  formed  a  bright  foil  to 
the  sombre  mass  of  rock  above  them.  The  sea  was  so 
calm  and  the  scene  so  lovely  that  Mr.  Huggins  and  my- 
self stayed  on  deck  till  near  midnight,  when  the  ship  was 
moored.  During  our  walking  to  and  fro  a  striking 
enlargement  of  the  disk  of  Jupiter  was  observed,  when- 
ever the  heated  air  of  the  funnel  came  between  us  and 
the  planet.  On  passing  away  from  the  heated  air,  the 
flat  dim  disk  would  immediately  shrink  to  a  luminous 
point.  The  effect  was  one  of  visual  persistence.  The 
retinal  image  of  the  planet  was  set  quivering  in  all 
azimuths  by  the  streams  of  heated  air,  describing  in 
quick  succession  minute  lines  of  light,  which  summed 
themselves  to  a  disk  of  sensible  area. 

At  six  o'clock  next  morning,  the  gun  at  the  Signal 
Station  on  the  summit  of  the  rock,  boomed.  At  eight 
the  band  on  board  the  'Trafalgar'  training-ship,  which 
was  in  the  harbour,  struck  up  the  national  anthem ;  and 
immediately  afterwards  a  crowd  of  mite-like  cadets 
swarmed  up  the  rigging.  After  the  removal  of  the 
apparatus  belonging  to  the  Gibraltar  party  we  went  on- 
shore. Winter  was  in  England  when  we  left,  but  here 
we  had  the  warmth  of  summer.  The  vegetation  was 
luxuriant— palm-trees,  cactuses,  and  aloes,  all  ablaze 
with  scarlet  flowers.  A  visit  to  the  Grovernor  was  pro- 
posed, as  an  act  of  necessary  courtesy,  and  I  accom- 
panied Admiral  Ommaney  and  Mr.  Huggins  to  *the 
Convent,'  or  Government  House.  We  sent  in  our  cards, 
waited  for  a  time,  and  were  then  conducted  by  an 
orderly  to  his  Excellency.  He  is  a  fine  old  man,  over 
six  feet  high,  and  of  frank  military  bearing.  He  re- 
ceived us  and  conversed  with  us  in  a  very  genial 
manner.  He  took  us  to  see  his  garden,  his  palms,  his 
shaded  promenades,  and  his  orange-trees  loaded  with 


I 


VOYAGE  TO  ALGERIA.  151 

fruit,  in  all  of  which  he  took  manifest  delight.  Evi- 
dently 'the  hejo  of  Kars'  had  fallen  upon  quarters 
after  his  own  heart.  He  appeared  full  of  good  nature, 
and  engaged  us  on  the  spot  to  dine  with  him  that  day. 

We  sought  the  town-major  for  a  pass  to  visit  the 
lines.  While  awaiting  his  arrival  I  purchased  a  stock 
of  white  glass  bottles,  with  a  view  to  experiments  on 
the  colour  of  the  sea.  Mr.  Huggins  and  myself,  who 
wished  to  see  the  rock,  were  taken  by  Captain  Salmond 
to  the  library,  where  a  model  of  Gibraltar  is  kept,  and 
where  we  had  a  useful  preliminary  lesson.  At  the 
library  we  met  Colonel  Maberly,  a  courteous  and 
kindly  man,  who  gave  us  good  advice  regarding  our 
excursion.  He  sent  an  orderly  with  us  to  the  entrance 
of  the  lines.  The  orderly  handed  us  over  to  an  intelli- 
gent Irishman,  who  was  directed  to  show  us  everything 
that  we  desired  to  see,  and  to  hide  nothing  from  us. 
We  took  the  '  upper  line,'  traversed  the  galleries  hewn 
through  the  limestone  ;  looked  through  the  embrasiures, 
which  opened  like  doors  in  the  precipice,  towards  the 
hills  of  Spain  ;  reached  St.  George's  hall,  and  went  still 
higher,  emerging  on  the  summit  of  one  of  the  noblest 
cliffs  I  have  ever  seen. 

Beyond  were  the  Spanish  lines,  marked  by  a  line  of 
white  sentry-boxes  ;  nearer  were  the  English  lines,  less 
conspicuously  indicated ;  and  between  both  was  the 
neutral  ground.  Behind  the  Spanish  lines  rose  the 
conical  hill  called  the  Queen  of  Spain's  Chair.  The 
general  aspect  of  the  mainland  from  the  rock  is  bold 
and  rugged.  Doubling  back  from  the  galleries,  we  struck 
upwards  towards  the  crest,  reached  the  Signal  Station, 
where  we  indulged  in  'shandy-gaff'  and  bread  and 
cheese.  Thence  to  O'Hara's  Tower,  the  highest  point  of 
the  rock.  It  was  built  by  a  former  Governor,  who,  for- 
getful of  the  laws  of  terrestrial  curvature,  thought  he 
11 


152  FRAGMENTS  OP  SCIENCE. 

might  look  from  the  tower  into  the  port  of  Cadiz.  The 
tower  is  riven,  and  it  may  be  climbed  along  the  edges 
of  the  crack.  We  got  to  the  top  of  it;  thence  de- 
scended the  curious  Mediterranean  Stair — a  zigzag, 
mostly  of  steps  down  a  steeply  falling  slope,  amid 
palmetto  brush,  aloes,  and  prickly  pear. 

Passing  over  the  Windmill  Hill,  we  were  joined  at 
the  *  Grovernor's  Cottage  '  by  a  car,  and  drove  afterwards 
to  the  lighthouse  at  Europa  Point.  The  tower  was 
built,  I  believe,  by  Queen  Adelaide,  and  it  contains  a 
fine  dioptric  apparatus  of  the  first  order,  constructed  by 
Messrs.  Chance,  of  Birmingham.  At  the  appointed 
hour  we  were  at  the  Convent.  During  dinner  the  same 
genial  traits  which  appeared  in  the  morning  were  still 
more  conspicuous.  The  freshness  of  the  Grovernor's 
nature  showed  itself  best  when  he  spoke  of  his  old  an- 
tagonist in  arms,  Mouraviefi.  Chivalry  in  war  is  con- 
sistent with  its  stern  prosecution.  These  two  men 
were  chivalrous,  and  after  striking  the  last  blow  became 
friends  for  ever.  Our  kind  and  courteous  reception  at 
Gibraltar  is  a  thing  to  be  remembered  with  pleasure. 

On  December  15  we  committed  ourselves  to  the 
Mediterranean.  The  views  of  Gibraltar  with  which  we 
are  most  acquainted  represent  it  as  a  huge  ridge  ;  but 
its  aspect,  end  on,  both  from  the  Spanish  lines  and  from 
the  other  side,  is  truly  noble.  There  is  a  sloping  bank 
of  sand  at  the  back  of  the  rock,  which  I  was  disposed 
to  regard  simply  as  the  debris  of  the  limestone.  I 
wished  to  let  myself  down  upon  it,  but  had  not  the 
time.  My  friend  Mr.  Busk,  however,  assures  me  that 
it  is  silica,  and  that  the  same  sand  constitutes  the  ad- 
jacent neutral  ground.  There  are  theories  afloat  as  to 
its  having  been  blown  from  Sahara.  The  Mediterranean 
throughout  this  first  day,  and  indeed  throughout  the 
entire  voyage  to  Oran,  was  of  a  less  deep  blue  than  the 


VOYAGE  TO  ALGERIA.  153 

Atlantic.  Possibly  the  quantity  of  organisms  may 
have  modified  the  colour.  At  night  the  phosphores- 
cence was  startling,  breaking  suddenly  out  along  the 
crests  of  the  waves  formed  by  the  port  and  starboard 
bows.  Its  strength  was  not  uniform.  Having  flashed 
brilliantly  for  a  time,  it  would  in  part  subside,  and 
afterwards  regain  its  vigour.  Several  large  phosphor- 
escent masses  of  weird  appearance  also  floated  past.         \ 

On  the  morning  of  the  16th  we  sighted  the  fort  and 
lighthouse  of  Marsa  el  Kibir,  and  beyond  them  the 
white  walls  of  Oran  lying  in  the  bight  of  a  bay,  shel- 
tered by  dominant  hills.  The  sun  was  shining  bright- 
ly ;  during  our  whole  voyage  we  had  not  had  so  fine  a 
day.  The  wisdom  which  had  led  us  to  choose  Oran  as 
our  place  of  observation  seemed  demonstrated.  A 
rather  excitable  pilot  came  on  board,  and  he  guided  us  in 
behind  the  Mole,  which  had  sufiered  much  damage 
the  previous  year  from  an  unexplained  outburst  of  waves 
from  the  Mediterranean.  Both  port  and  bow  anchors 
were  cast  in  deep  water.  With  three  huge  hawsers  the 
ship's  stern  was  made  fast  to  three  gun-pillars  fixed  in 
the  Mole ;  and  here  for  a  time  the  « Urgent '  rested 
from  her  labours. 

M.  Janssen,  who  had  rendered  his  name  celebrated 
by  his  observations  of  the  eclipse  in  India  in  1868, 
when  he  showed  the  solar  flames  to  be  eruptions  of  in- 
candescent hydrogen,  was  already  encamped  in  the  open 
country  about  eight  miles  from  Oran.  On  December  2 
he  had  quitted  Paris  in  a  balloon,  with  a  strong  young 
sailor  as  his  assistant,  had  descended  near  the  mouth  of 
the  Loire,  seen  M.  Grambetta,  and  received  from  him 
encouragement  and  aid.  On  the  day  of  our  arrival  his 
encampment  was  visited  by  Mr.  Huggins,  and  the  kind 
and  courteous  Engineer  of  the  Port  drove  me  subse- 
quently, in  his  own  phaeton,  to  the  place.     It  bore  the 


154  ,     FRAGMENTS  OF  SCIENCE. 

best  repute  as  regards  freedom  from  haze  and  fog,  and 
commanded  an  open  outlook  ;  but  it  was  inconvenient 
for  us  on  account  of  its  distance  from  the  ship.  The 
place  next  in  repute  was  the  railway  station,  between 
two  and  three  miles  distant  from  the  Mole.  It  was 
inspected,  but,  being  enclosed,  was  abandoned  for  an 
eminence  in  an  adjacent  garden,  the  property  of  Mr. 
Hinshelwood,  a  Scotchman  who  had  settled  some  years 
previously  as  an  Esparto  merchant  in  Oran.*  He,  in 
the  most  liberal  manner,  placed  his  ground  at  the  dis- 
position of  the  party.  Here  the  tents  were  pitched,  on 
the  Saturday,  by  Captain  Salmond  and  his  intelligent 
corps  of  sappers,  the  instruments  being  erected  on  the 
Monday  under  cover  of  the  tents. 

Close  to  the  railway  station  runs  a  new  loopholed 
wall  of  defence,  through  which  the  highway  passes  into 
the  open  country.  Standing  on  the  highway,  and 
looking  southwards,  about  twenty  yards  to  the  right  is 
a  small  bastionet,  intended  to  carry  a  gun  or  two.  Its 
roof  I  thought  would  form  an  admirable  basis  for  my 
telescope,  while  the  view  of  the  surrounding  country 
was  unimpeded  in  all  directions.  The  authorities 
kindly  allowed  me  the  use  of  this  bastionet.  Two 
men,  one  a  blue-jacket  named  Elliot,  and  the  other  a 
marine  named  Hill,  were  placed  at  my  disposal  by 
Lieutenant  Walton ;  and,  thus  aided,  on  Monday  morn- 
ing I  mounted  my  telescope.  The  instrument  was 
dew  to  me,  and  some  hours  of  discipline  were  spent  in 
mastering  all  the  details  of  its  manipulation. 

Mr.  Huggins  joined  me,  and  we  visited  together  the 
Aidb  quarter  of  Oran.  The  flat-roofed  houses  appeared 
very  clean  and  white.  The  street  was  filled  with 
loiterers,  and  the  thresholds  were  occupied  by  pictur- 

*  Esparto  is  a  kind  of  grass  now  much  used  in  the  manufacture 
of  paper. 


VOYAGE  TO   ALGERIA.  155 

esque  groups.  Some  of  the  men  were  very  fine.  We 
saw  many  straight,  manly  fellows  who  must  have  been 
six  feet  four  in  height.  They  passed  us  with  perfect 
indifference,  evincing  no  anger,  suspicion,  or  curiosity, 
hardly  caring  in  fact  to  glance  at  us  as  we  passed.  In 
one  instance  only  during  my  stay  at  Oran  was  I  spoken 
to  by  an  Arab.  He  was  a  tall,  good-humoured  fellow, 
who  came  smiling  up  to  me,  and  muttered  something 
about  '  les  Anglais.'  The  mixed  population  of  Oran  is 
picturesque  in  the  highest  degree  :  the  Jews,  rich  and 
poor,"  varying  in  their  costumes  as  their  wealth  varies  ; 
the  Arabs  more  picturesque  still,  and  of  all  shades  of 
complexion — the  negroes,  the  Spaniards,  the  French, 
all  grouped  together,  each  race  preserving  its  own  indi- 
viduality, formed  a  picture  intensely  interesting  to  me. 
On  Tuesday,  the  20th,  I  was  early  at  the  bastionet. 
The  night  had  been  very  squally.  The  sergeant  of  the 
sappers  had  taken  charge  of  our  key,  and  on  Tuesday 
morning  Elliot  went  for  it.  He  brought  back  the  intelli- 
gence that  the  tents  had  been  blown  down,  and  the 
instruments  overturned.  Among  these  was  a  large  and 
valuable  equatorial  from  the  Eoyal  Observatory,  Green- 
wich. It  seemed  hardly  possible  that  this  instrument, 
with  its  wheels  and  verniers  and  delicate  adjustments^ 
could  have  escaped  uninjured  from  such  a  fall.  This, 
however,  was  the  case  ;  and  during  the  day  all  the  over- 
turned instruments  were  restored  to  their  places,  and 
found  to  be  in  practical  working  order.  This  and  the 
following  day  were  devoted  to  incessant  schooling.  I 
had  come  out  as  a  general  stargazer,  and  not  with  the 
intention  of  devoting  myself  to  the  observation  of  any 
particular  phenomenon.  I  wished  to  see  the  whole — 
the  first  contact,  the  advance  of  the  moon,  the  suc- 
cessive swallowing  up  of  the  solar  spots,  the  breaking 
of  the  last  line  of  crescent  by  the  lunai-  mountains  into 


156  FKAGMENT3  OF  SCIENCE. 

Bailey's  beads,  the  advance  of  the  shadow  through  the 
air,  the  appearance  of  the  corona  and  promiaences  at 
the  moment  of  totality,  the  radiant  streamers  of  the 
corona,  the  internal  structure  of  the  flames,  a  glance 
through  a  polariscope,  a  sweep  round  the  landscape 
with  the  naked  eye,  the  reappearance  of  the  solar  limb 
through  Bailey's  beads,  and,  finally,  the  retreat  of  the 
lunar  shadow  through  the  air. 

I  was  provided  with  a  telescope  of  admirable  defi- 
nition, mounted,  adjusted,  packed,  and  most  liberally 
placed  at  my  disposal  by  Mr.  Warren  De  La  Eue.  The 
telescope  grasped  the  whole  of  the  sun,  and  a  consider- 
able portion  of  the  space  surrounding  it.  But  it  would 
not  take  in  the  extreme  limits  of  the  corona.  For  this 
I  had  lashed  on  to  the  large  telescope  a  light  but 
powerful  instrument,  constructed  by  Eoss,  and  lent  to 
me  by  Mr.  Huggins.  I  was  also  furnished  with  an  ex- 
cellent binocular  by  Mr.  Dallmeyer.  In  fact,»no  man 
could  have  been  more  efficiently  supported.  It  required 
a  strict  parcelling  out  of  the  interval  of  totality  to  em- 
brace in  it  the  entire  series  of  observations.  These, 
while  the  sun  remained  visible,  were  to  be  made  with 
an  unsilvered  diagonal  eye-piece,  which  reflected  but  a 
small  fraction  of  the  sun's  ligbt,  this  fraction  being 
still  further  toned  down  by  a  dark  glass.  At  the 
moment  of  totality  the  dark  glass  was  to  be  removed, 
and  a  silver  reflector  pushed  in,  so.  as  to  get  the  maxi- 
mum of  light  from  the  corona  and  prominences.  The 
time  of  totality  was  distributed  as  follows : 


1.  Observe  approach  of  shadow  through  the  air .  totality, 

2.  Telescope  .  .  .  .  30  seconds. 
.  30  seconds. 
.  15  seconds. 
.  10  sicondS: 
.  20  seconds. 
.  20  seconds. 


3.  Finder 

4.  Double  image  prism 
6.  Naked  eye   . 

6.  Finder  or  binocular 

7.  Telescope     . 


8i  Observe  retreat  of  shadow. 


VOYAGE  TO  ALGERIA.  157 

lu  our  rehearsals  Elliot  stood  beside  me,  watch  in 
hand,  and  furnished  with  a  lantern.  He  called  out  at 
the  end  of  each  interval,  while  I  moved  from  telescope 
to  finder,  from  finder  to  polariscope,  from  polariscope 
to  naked  eye,  from  naked  eye  back  to  finder,  from 
finder  to  telescope,  abandoning  the  instrument  finally 
to  observe  the  retreating  shadow.  All  this  we  went 
over  twenty  times,  while  looking  at  the  actual  sun,  and 
keeping  him  in  the  middle  of  the  field.  It  was  my 
object  to  render  the  repetition  of  the  lesson  so  mechan- 
ical as  to  leave  no  room  for  flurry,  forgetfulness,  or 
excitement.  Volition  was  not  to  be  called  upon,  nor 
judgment  exercised,  but  a  well-beaten  path  of  routine 
was  to  be  followed.  Had  the  opportunity  occurred,  I 
think  the  programme  would  have  been  strictly  carried 
out. 

But  the  opportunity  did  not  occur.  For  several  days 
the  weather  had  been  ill-natured.  We  had  wind  so  strong 
as  to  render  the  hawsers  at  the  stern  of  the  '  Urgent '  as 
rigid  as  iron,  and  to  destroy  the  navigating  lieutenant's 
sleep.  We  had  clouds,  a  thunder-storm,  and  some  rain. 
Still  the  hope  was  held  out  that  the  atmosphere  would 
cleanse  itself,  and  if  it  did  we  were  promised  air  of 
extraordinary  limpidity.  Early  on  the  22nd  we  were 
all  at  our  posts.  Spaces  of  blue  in  the  early  morning 
gave  us  some  encouragement,  but  all  depended  on  the 
relation  of  these  spaces  to  the  surrounding  clouds. 
Which  of  them  were  to  grow  as  the  day  advanced  ? 
The  wind  was  high,  and  to  secm-e  the  steadiness  of  my 
instrument  I  was  forced  to  retreat  behind  a  projection 
of  the  bastionet,  place  stones  upon  its  stand,  and, 
further,  to  avail  myself  of  the  shelter  of  a  sail.  My 
practised  men  fastened  the  sail  at  the  top,  and  loaded 
it  with  boulders  at  the  bottom.  It  was  tried  severely, 
but  it  stood  firm. 


158  FRAGMENTS  OF  SCIENCfE. 

The  clouds  and  blue  spaces  fought  for  a  time  with 
varying  success.  The  sun  was  hidden  and  revealed  at 
intervals,  hope  oscillating  in  synchronism  with  the 
changes  of  the  sky.  At  the  moment  of  first  contact  a 
dense  cloud  intervened;  but  a  minute  or  two  afterwards 
the  cloud  had  passed,  and  the  encroachment  of  the 
black  body  of  the  moon  was  evident  upon  the  solar 
disk.  The  moon  marched  onward,  and  I  saw  it  at 
frequent  intervals;  a  large  group  of  spots  were  ap- 
proached and  swallowed  up.  Subsequently  I  caught 
sight  of  the  lunar  limb  as  it  cut  through  the  middle  of 
a  large  spot.  The  spot  was  not  to  be  distinguished  from 
the  moon,  but  rose  like  a  mountain  above  it.  The 
clouds,  when  thin,  could  be  seen  as  grey  scud  drifting 
across  the  black  surface  of  the  moon ;  but  they  thick- 
ened more  and  more,  and  made  the  intervals  of  clear- 
ness scantier.  During  these  moments  I  watched  with 
an  interest  bordering  upon  fascination  the  march  of  the 
silver  sickle  of  the  sun  across  the  field  of  the  telescope. 
It  was  so  sharp  and  so  beautiful.  No  trace  of  the  lunar 
limb  could  be  observed  beyond  the  sun's  boundary.  Here, 
indeed,  it  could  only  be  relieved  by  the  corona,  which 
was  utterly  cut  off  by  the  dark  glass.  The  blackness  of 
the  moon  beyond  the  sun  was,  in  fact,  confounded  with 
the  blackness  of  space. 

Beside  me  was  Elliot  with  the  watch  and  lantern, 
while  Lieutenant  Archer,  of  the  Eoyal  Engineers,  had 
the  kindness  to  take  charge  of  my  note-book.  I  men- 
tioned, and  he  wrote  rapidly  down,  such  things  as 
seemed  worthy  of  remembrance.  Thus  my  hands  and 
mind  were  entirely  free ;  but  it  was  all  to  no  purpose. 
A  patch  of  sunlight  fell  and  rested  upon  the  landscape 
some  miles  away.  It  was  the  only  illuminated  spot 
within  view.  But  to  the  north-west  there  was  still  a 
space  of  blue  which  might  reach  us  in  time.     Within 


VOYAGE  TO  ALGERIA.  159 

Reven  minutes  of  totality  another  space  towards  the 
zenith  became  very  dark.  The  atmosphere  was,  as  it 
were,  on  the  brink  of  a  precipice,  being  charged  with 
humidity,  which  required  but  a  slight  chill  to  bring  it 
down  in  clouds.  This  was  furnished  by  the  withdrawal 
of  the  solar  beams :  the  clouds  did  come  down,  cover- 
ing up  the  space  of  blue  on  which  our  hopes  had  so 
long  rested.  I  abandoned  the  telescope  and  walked  to 
and  fro  in  despair.  As  the  moment  of  totality  ap- 
proached, the  descent  towards  darkness  was  as  obvious 
as  a  falling  stone.  I  looked  towards  a  distant  ridge, 
where  the  darkness  would  first  appear.  At  the  moment 
a  fan  of  beams,  issuing  from  the  hidden  sun,  was  spread 
out  over  the  southern  heavens.  These  beams  are  bars 
of  alternate  light  and  shade,  produced  in  illuminated 
haze  by  the  shadows  of  floating  cloudlets  of  varying 
density.  The  beams  are  practically  parallel,  but  by  an 
effect  of  perspective  they  appear  divergent,  having  the 
sun,  in  fact,  for  their  point  of  convergence.  The  dark- 
ness took  possession  of  the  ridge  referred  to,  lowered 
upon  M.  Janssen's  observatory,  passed  over  the  southej*n 
heavens,  blotting  out  the  beams  as  if  a  sponge  had 
been  drawn  across  them.  It  then  took  successive  pos- 
session of  three  spaces  of  blue  sky  in  the  south-eastern 
atmosphere.  I  again  looked  towards  the  ridge.  A 
glimmer  as  of  day-dawn  was  behind  it,  and  immediately 
afterwards  the  fan  of  beams,  which  had  been  for  more 
than  two  minutes  absent,  revived.  The  eclipse  of  1870 
had  ended,  and,  as  far  as  the  corona  and  flames  were 
concerned,  we  had  been  defeated. 

Even  in  the  heart  of  the  eclipse  the  darkness  was  by 
no  means  perfect.  Small  print  could  be  read.  In  fact, 
the  clouds  which  rendered  the  day  a  dark  one,  by  scat- 
tering light  into  the  shadow,  rendered  the  darkness  less 
intense  than  it  would  have  been  had  the  atmosphere  been 


160  FEAaMENTS  OF  SCIENCE. 

without  cloud.  In  the  more  open  spaces  I  soup^ht  for 
stars,  but  could  find  none.  There  was  a  lull  in  the  wind 
before  and  after  totality,  but  during  the  totality  the  wind 
was  strong.  I  waited  for  some  time  on  the  bastionet, 
hoping  to  get  a  glimpse  of  the  moon  on  the  opposite 
border  of  the  sun,  but  in  vain.  The  clouds  continued, 
and  some  rain  fell.  The  day  brightened  somewhat  after- 
wards, and,  having  packed  all  up,  in  the  sober  twilight 
Mr.  Crookes  and  myself  climbed  the  heights  above  the 
fort  of  Vera  Cruz.  From  this  eminence  we  had  a  very 
noble  view  over  the  Mediterranean  and  the  flanking 
African  hills.  The  sunset  was  remarkable,  and  the  whole 
outlook  exceedingly  fine. 

The  able  and  well-instructed  medical  officer  of  the 
*  Urgent,'  Mr.  Groodman,  observed  the  following  tem- 
peratures during  the  progress  of  the  eclipse  : 


Hour 

Deg. 

Hour 

Deg 

11.46        . 

.      66 

12.43 

.      61 

11.55 

65 

1.6 

.      62 

12.10 

64 

1.27 

.      63 

12.37 

63 

1.44 

.      66 

12.39        . 

52 

2.10 

.      57 

The  minimum  temperature  occurred  some  minutes 
after  totality,  when  a  slight  rain  fell. 

The  wind  was  so  strong  on  the  23rd  that  Captain 
Henderson  would  not  venture  out.  Guided  by  Mr. 
Groodman,  I  visited  a  cave  in  a  remarkable  stratum 
of  shell-breccia,  and,  thanks  to  my  guide,  secured  speci- 
mens. Mr.  Busk  informs  me  that  a  precisely  similar 
breccia  is  found  at  Gribraltar,  at  approximately  the 
same  level.  During  the  afternoon,  Admiral  Ommaney 
and  myself  drove  to  the  fort  of  Marsa  el  Kibir.  The 
fortification  is  of  ancient  origin,  the  Moorish  arches  being 
still  there  in  decay,  but  the  fort  is  now  very  strong. 
A.bout  four  or  five  hundred  fine-looking  dragoons  were 


VOYAGE  TO  ALGERIA.  161 

looking  after  their  horses,  waiting  for  a  lull  to  enable 
them  to  embark  for  France.  One  of  their  officers  was 
wandering  in  a  very  solitary  fashion  over  the  fort.  We 
had  some  conversation  with  him.  He  had  been  at 
Sedan,  had  been  taken  prisoner,  but  had  effected  his 
escape.  He  shook  his  head  when  we  spoke  of  the  ter- 
mination of  the  war,  and  predicted  its  long  continuance. 
There  was  bitterness  in  his  tone  as  he  spoke  of  the 
charges  of  treason  so  lightly  levelled  against  French 
commanders.  The  green  waves  raved  round  the  pro- 
montory on  which  the  fort  stands,  smiting  the  rocks, 
breaking  into  foam,  and  jumping,  after  impact,  to  a 
heiofht  of  a  hundred  feet  and  more  into  the  air-  As  we 
returned  our  vehicle  broke  down  through  the  loss  of  a 
wheel.  The  Admiral  went  on  board,  while  I  remained 
long  watching  the  agitated  sea.  The  little  horses  of 
Oran  well  merit  a  passing  word.  Their  speed  and  en- 
durance, both  of  which  are  heavily  drawn  upon  by  their 
drivers,  are  extraordinary. 

The  wind  sinking,  we  lifted  anchor  on  the  24th. 
For  some  hours  we  went  pleasantly  along ;  but  during 
the  afternoon  the  storm  revived,  and  it  blew  heavily 
against  us  all  the  night.  When  we  came  opposite  the 
Bay  of  Almeria,  on  the  25th,  the  captain  turned  the 
ship,  and  steered  into  the  bay,  where,  under  the  shadow 
of  the  Sierra  Nevada,  we  passed  Christmas  night  in 
peace.  Next  morning  '  a  rose  of  dawn '  rested  on  the 
snows  of  the  adjacent  mountains,  while  a  purple  haze 
was  spread  over  the  lower  hills.  I  had  no  notion  that 
Spain  possessed  so  fine  a  range  of  mountains  as  the 
Sierra  Nevada.  The  height  is  considerable,  but  the 
form  also  is  such  as  to  get  the  maximum  of  grandeur 
out  of  the  height.  We  weighed  anchor  at  eight  a.m., 
passing  for  a  time  through  shoal  water,  the  bottom 
having  been  evidently  stirred  up.     The  adjacent  land 


162  FKAGMENTS  OF  SCIENCE. 

seemed  eroded  in  a  remarkable  manner.  It  has  its 
floods,  which  excavate  these  valleys  and  ravines,  and 
leave  those  singular  ridges  behind.  Towards  evening  I 
climbed  the  mainmast,  and,  standing  on  the  cross-trees, 
saw  the  sun  set  amid  a  blaze  of  fiery  clouds.  The  wind 
was  strong  and  bitterly  cold,  and  I  was  glad  to  slide 
back  to  the  deck  along  a  rope,  which  stretched  from 
the  mast-head  to  the  ship's  side.  That  night  we  cast 
anchor  beside  the  Mole  of  Gribraltar. 

On  the  morning  of  the  27th,  in  company  with  two 
friends,  I  drove  to  the  Spanish  lines,  with  the  view  of 
seeing  the  rock  from  that  side.  It  is  an  exceedingly 
noble  mass.  The  Peninsular  and  Oriental  mail-boat 
had  been  signalled  and  had  come.  Heavy  duties  called 
me  homeward,  and  by  transferring  myself  from  the 
'  Urgent '  to  the  mail-steamer  I  should  gain  three  days. 
I  hired  a  boat,  rowed  to  the  steamer,  learned  that  she 
was  to  start  at  one,  and  returned  with  all  speed  to  the 
'  Urgent.'  Making  known  to  Captain  Henderson  my 
wish  to  get  away,  he  expressed  doubts  as  to  the  pos- 
sibility of  reaching  the  mail-steamer  in  time.  With  his 
accustomed  kindness,  be  however  placed  a  boat  at  my 
disposal.  Four  hardy  fellows  and  one  of  the  ship's  officers 
jumped  into  it;  my  luggage,  hastily  thrown  together, 
was  tumbled  in,  and  we  were  immediately  on  our  way. 
We  had  nearly  four  miles  to  row  in  about  twenty 
minutes ;  but  we  hoped  the  mail-boat  might  not  be 
punctual.  For  a  time  we  watched  her  anxiously  ;  there 
was  no  motion ;  we  came  nearer,  but  the  flags  were  not 
yet  hauled  in.  The  men  put  forth  all  their  strength, 
animated  by  the  exhortations  of  the  officer  at  the  helm. 
The  roughness  of  the  sea  rendered  their  eflbrts  to  some 
extent  nugatory :  still  we  were  rapidly  approaching  the 
steamer.  At  length  she  moved,  punctual  almost  to  the 
minute,  at  first  slowly,  but  soon  with  quickened  pace. 


VOYAGE  TO  ALGEKIA.  163 

We  turned  to  the  left,  so  as  to  cut  across  her  bows. 
Five  minutes'  pull  would  have  brought  us  up  to  her. 
The  officer  waved  his  cap  and  I  my  hat.  '  If  they  could 
only  see  us,  they  might  back  to  us  in  a  moment.'  But 
they  did  not  see  us,  or  if  they  did,  they  paid  us  no  at- 
tention. I  returned  to  the  '  Urgent,'  discomfited,  but 
grateful  to  the  fine  fellows  who  had  wrought  so  hard  to 
carry  out  my  wishes. 

Glad  of  the  quiet,  in  the  sober  afternoon  I  took  a 
walk  towards  Europa  Point.  The  sky  darkened  and 
heavy  squalls  passed  at  intervals.  Private  theatricals 
were  at  the  Convent,  and  the  kind  and  courteous  Go- 
vernor had  sent  cards  to  the  eclipse  party.  I  failed  in 
my  duty  in  nnt  going.  St.  Michael's  Cave  is  said  to  rival, 
if  it  does  not  outrival,  the  Mammoth  Cave  of  Kentucky. 
On  the  28th  Mr.  Crookes,  Mr.  Carpenter,  and  myself, 
guided  by  a  military  policeman  who  understood  his  work, 
explored  the  cavern.  The  mouth  is  about  1,100  feet 
above  the  sea.  We  zigzagged  up  to  it,  and  first  were 
led  into  an  aperture  in  the  rock,  at  some  height  above 
the  true  entrance  of  the  cave.  In  this  upper  cavern 
we  saw  some  tall  and  beautiful  stalactite  pillars. 

The  water  drips  from  the  roof  charged  with  bicar- 
bonate of  lime.  Exposed  to  the  air,  the  carbonic  acid 
partially  escapes,  and  the  simple  carbonate  of  lime, 
which  is  hardly  at  all  soluble  in  water,  deposits  itself  as 
a  solid,  forming  stalactites  and  stalagmites.  Even  the 
exposure  of  chalk  or  limestone  water  to  the  open  air 
partially  softens  it.  A  specimen  of  the  Eedboume 
water  exposed  by  Professors  Graham,  Miller,  and  Hof- 
mann,  in  a  shallow  basin,  fell  from  eighteen  degrees  to 
nine  degrees  of  hardness.  The  softening  process  of  Clark 
is  virtually  a  hastening  of  the  natural  process.  Here, 
however,  instead  of  being  permitted  to  evaporate,  half 
the    carbonic    acid  is  appropriated  by  lime,  the  half 


164  FRAGMENTS  OF  SCIENCE. 

thus  taken  up,  as  well  as  the  remaining  half,  being 
precipitated.  The  solid  precipitate  is  permitted  to 
sink,  and  the  clear  supernatant  liquid  is  limpid  soft 
water. 

We  returned  to  the  real  mouth  of  St.  Michael's 
Cave,  which  is  entered  by  a  wicket.  The  floor  was 
somewhat  muddy,  and  the  roof  and  walls  were  wet.  We 
soon  found  ourselves  in  the  midst  of  a  natural  temple, 
where  tall  columns  sprang  complete  from  floor  to  roof, 
while  incipient  columns  were  growing  to  meet  each  other, 
upwards  and  downwards.  The  water  which  trickles 
from  the  stalactite,  after  having  in  part  yielded  up  its 
carbonate  of  lime,  falls  upon  the  floor  vertically  under- 
neath, and  there  builds  the  stalagmite.  Consequently, 
the  pillars  grow  from  above  and  below  simultaneously, 
along  the  same  vertical.  It  is  easy  to  distinguish  the 
fitalagmitic  from  the  stalactitic  portion  of  the  pillars. 
The  former  is  always  divided  into  short  segments  by 
protuberant  rings,  as  if  deposited  periodically,  while 
the  latter  presents  a  uniform  surface.  In  some  cases 
the  points  of  inverted  cones  of  stalactite  rested  on  the 
centres  of  pillars  of  stalagmite.  The  process  of  solidi- 
fication and  the  consequent  architectmre  were  alike 
beautiful. 

We  followed  our  guide  through  various  branches  and 
arms  of  the  cave,  climbed  and  descended  steps,  halted 
at  the  edges  of  dark  shafts  and  apertures,  and  squeezed 
ourselves  through  narrow  passages.  From  time  to  time 
we  halted,  while  Mr.  Crookes  illuminated  with  ignited 
magnesium  wire,  the  roof,  columns,  dependent  spears, 
and  graceful  drapery  of  the  stalactites.  Once,  coming 
to  a  magnificent  cluster  of  icicle-like  spears,  we  helped 
ourselves  to  specimens.  There  was  some  difficulty  in 
detaching  the  more  delicate  ones,  their  fragility  was  so 
great.     A  consciousness  of  vandalism,  which  smote  nie 


VOYAGE  TO  ALGERIA.  165 

at  the  time,  haunts  me  still ;  for,  though  our  requisi- 
tions were  moderate,  this  beauty  ought  not  to  be  at  all 
invaded.  Pendent  from  the  roof,  in  their  natural 
habitat,  nothing  can  exceed  their  delicate  beauty ;  they 
live,  as  it  were,  surrounded  by  organic  connections.  In 
London  they  are  curious,  but  not  beautiful.  Of  gathered 
shells  Emerson  writes : 

I  wiped  away  the  weeds  and  foam, 
And  brought  my  sea-born  treasures  home : 
But  the  poor,  unsightly,  noisome  things 
Had  left  their  beauty  on  the  shore, 
With  the  smi,  and  the  sand,  and  the  wild  uproar. 

The  promontory  of  Gibraltar  is  so  burrowed  with 
caverns  that  it  has  been  called  the  Hill  of  Caves.  They 
are  apparently  related  to  the  geologic  disturbances 
which  the  rock  has  undergone.  The  earliest  of  these  is 
the  tilting  of  the  once  horizontal  strata.  Suppose  a 
force  of  torsion  to  act  upon  the  promontory  at  its 
southern  extremity  near  Europa  Point,  and  suppose  the 
rock  to  be  of  a  partially  yielding  character;  such  a 
force  would  twist  the  strata  into  screw-surfaces,  the 
greatest  amount  of  twisting  being  endured  near  the 
point  of  application  of  the  force.  Such  a  twisting  the 
rock  appears  to  have  suffered ;  but  instead  of  the  twist 
fading  gradually  and  uniformly  off,  in  passing  from 
south  to  north,  the  want  of  uniformity  in  the  material 
has  produced  lines  of  dislocation  where  there  are  abrupt 
changes  in  the  amount  of  twist.  Thus,  at  the  northern 
end  of  the  rock  the  dip  to  the  west  is  nineteen  degrees; 
in  the  Middle  Hill,  it  is  thirty-eight  degrees ;  in  the 
centre  of -the  South  hill,  or  Sugar  Loaf,  it  is  tifty-seven 
degrees.  At  the  southern  extremity  of  the  Sugar  Loaf 
the  strata  are  vertical,  while  farther  to  the  south  they 
jvctually  turn  over  and  dip  to  the  east. 

The  rock  is  thus  divided  into  three  sections,  sepa- 


1G6  FRAGMENTS  OF  SCIENCE. 

rated  from  each  other  by  places  of  dislocation,  where  the 
strata  are  much  wrenched  and  broken.  These  are 
called  the  Northern  and  Southern  Quebrada,  from  the 
Spanish  '  Tierra  Quebrada,'  or  broken  ground.  It  is  at 
these  places  that  the  inland  caves  of  Gibraltar  are 
almost  exclusively  found.  Based  on  the  observations  of 
Dr.  Falconer  and  himself,  an  excellent  and  most  in- 
teresting account  of  these  caves,  and  of  the  human 
remains  and  works  of  art  which  they  contain,  was  com- 
municated by  Mr.  Busk  to  the  meeting  of  the  Congress 
of  Prehistoric  Archaeology  at  Norwich,  and  afterwards 
printed  in  the  '  Transactions  '  of  the  Congress.*  Long 
subsequent  to  the  operation  of  the  twisting  force  just 
referred  to,  the  promontory  underwent  various  changes 
of  level.  There  are  sea-terraces  and  layers  of  shell- 
breccia  along  its  flanks,  and  numerous  caves  which,  unlike 
the  inland  ones,  are  the  product  of  marine  erosion.  The 
Ape's  Hill,  on  the  African  side  of  the  strait,  Mr.  Busk 
informs  me  has  undergone  similar  disturbances.' 

In  the  harbour  of  Gribraltar,  on  the  morning  of  our 
departure,  I  resumed  a  series  of  observations  on  the 
colour  of  the  sea.  On  the  way  out  a  number  of 
specimens  had  been  collected,  with  a  view  to  subsequent 
examination.  But  the  bottles  were  claret  bottles,  of 
doubtful  purity.  At  Gribraltar,  therefore,  I  purchased 
fifteen  white  glass  bottles,  with  ground  glass  stoppers, 
and  at  Cadiz,  thanks  to  the  friendly  guidance  of  Mr. 
Cameron,  I  secured  a  dozen  more.     These  seven-and- 

'  In  this  essay  Mr.  Busk  refers  to  the  previous  labours  of  Mr. 
Smith,  of  Jordan  Hill,  to  whom  we  owe  most  of  our  knowledge  of 
the  geology  of  the  rock. 

'  No  one  can  rise  from  the  perusal  of  Mr.  Busk's  paper  without 
a  feeling  of  admiration  for  the  principal  discoverer  and  indefa- 
tigable explorer  of  the  Gibraltar  caves,  the  late  Captain  Frederiols 
Brome. 


VOYAGE  TO  ALGERIA.  167 

twenty  bottles  were  filled  with  water,  taken  at  different 
places  between  Oran  and  Spithead. 

And  here  let  me  express  my  warmest  acknowledg- 
ments to  Captain  Henderson,  the  commander  of  H.M.S. 
*  Urgent,'  who  aided  me  in  my  observations  in  every 
possible  way.  Indeed,  my  thanks  are  due  to  all  the 
officers  for  their  unfailing  courtesy  and  help.  The 
captain  placed  at  my  disposal  his  own  coxswain,  an 
intelligent  fellow  named  Thorogood,  who  skilfully 
attached  a  cord  to  each  bottle,  weighted  it  with  lead, 
cast  it  into  the  sea,  and,  after  three  successive  rinsings, 
filled  it  under  my  own  eyes.  The  contact  of  jugs, 
buckets,  or  other  vessels  was  thus  avoided ;  and  even 
the  necessity  of  pouring  out  the  water,  afterwards, 
through  the  dirty  London  air. 

The  mode  of  examination  applied  to  these  bottles 
has  been  already  described.^  The  liquid  is  illuminated 
by  a  powerfully  condensed  beam,  its  condition  being 
revealed  through  the  light  scattered  by  its  suspended 
particles.  '  Care  is  taken  to  defend  the  eye  from  the 
access  of  all  other  light,  and,  thus  defended,  it  becomes 
an  organ  of  inconceivable  delicacy.'  '  Were  water  of 
uniform  density  perfectly  free  from  suspended  matter, 
it  would,  in  my  opinion,  scatter  no  light  at  all.  Tlie 
track  of  a  luminous  beam  could  not  be  seen  in  such 
water.  But  '  an  amount  of  impurity  so  infinitesimal  as 
to  be  scarcely  expressible  in  numbers,  and  the  individual 
particles  of  which  are  so  small  as  wholly  to  elude  the 
microscope,  may,  when  examined  by  the  method  alluded 
to,  produce  not  only  sensible,  but  striking,  effects  upon 
the  eye.' 

The  results  of  the  examination  of  nineteen  bottles 
filled  at  various  places  between  Gribraltar  and  Spithead 
are  here  tabulated : 

*  'Floating  Matter  of  the  Air.'  Art.  *Dust  and  Disease.' 
12 


168 


FEAGMENTS  OF  SCIENCE. 


No. 

Locality 

Colour  of  Sea 

Appearance  in  Luminous  Beam 

1 

Gibraltar  Harbour .    .    . 

Green     .    .    . 

Thick  with  fine  particles 

2 

Two  miles  from  Gibraltar 

Clearer  green  . 

Thick  with  very  fine  particles 

8 

Off  Cabreta  Point    .    .    . 

Bright  green  . 

Still  thick,  but  less  so 

4 

OfE  Cabreta  Point  .    .    . 

Black-indigo  . 

Much  less  thick,  very  pire 

6 

Off  Tarifa 

Undecided  .    . 

Thicker  than  No.  4 

6 

Beyond  Tarif  a     .... 

Cobalt-blue     . 

Much  purer  than  No.  6 

7 

Twelve  miles  from  Cadiz . 

Yellow-green  . 

Very  thick 

8 

Cadiz  Harbour    .... 

Yellow-green  . 

Exceedingly  thick 

9 

Fourteen  miles  from  Cadiz 

Yellow-green  . 

Thick,  but  less  so 

10 

Fourteen  miles  from  Cadiz 

Bright  green  . 

Much  less  thick 

11 

Between  Capes  St.  Mary 

and  Vincent  .... 

Deep  indigo     . 

Very  little  matter,  very  pure 

12 

Off  theBmiings.    . 

Strong  green  . 

Thick,  with  fine  matter 

13 

Beyond  the  Burlings 

Indigo    .    .    . 

Very  Uttle  matter,  pure 

14 

Off  Cape  Finist.erre. 

Undecided  .    . 

Less  pure 

15 

Bay  of  Biscay     .    , 

Black-indigo  . 

Very  little  matter,  very  pure 

16 

Bay  of  Biscay     .    .    , 

Indigo    .    .    . 

Very  fine  matter.    Iridescent 

17 
18 

OffUshant      .    .    . 

Dark  green .    . 
Yellow-green  . 

A  good  deal  of  matter 
Exceedingly  thick 

Off  St.  Catherine's  . 

19 

Spithead 

Green     .    .    . 

Exceedingly  thick 

Here  we  have  three  specimens  of  water,  described  as 
green,  a  clearer  green,  and  bright  green,  taken  in 
Gribraltar  Harbour,  at  a  point  two  miles  from  the 
harbour,  and  off  Cabreta  Point.  The  home  examina- 
tion showed  the  first  to  be  thick  with  suspended  matter, 
the  second  less  thick,  and  the  third  still  less  thick. 
Thus  the  green  brightened  as  the  suspended  matter 
diminished  in  amount. 

Previous  to  the  fourth  observation  our  excellent 
navigating  lieutenant,  Mr.  Brown,  steered  along  the 
coast,  thus  avoiding  the  adverse  current  which  sets  in, 
through  the  Strait,  from  the  Atlantic  to  the  Mediter- 
ranean. He  was  at  length  forced  to  cross  the  boundary 
of  the  Atlantic  current,  which  was  defined  with  extra- 
ordinary sharpness.  On  the  one  side  of  it  the  water 
was  a  vivid  green,  on  the  other  a  deep  blue.  Standing 
at  the  bow  of  the  ship,  a  bottle  could  be  filled  with 
blue  water,  while  at  the  same  moment  a  bottle  cast 
from  the  stern  could  be  filled  with  green  water.  Two 
bottles  were  secured,  one  on  each  side  of  this  remarkable 
boundary.     In  the  distance  the  Atlantic  had  the  hue 


I 


VOYAGE  TO  ALGEKIA.  169 

called  ultra-marine ;  but  looked  fairly  down  upon,  it 
was  of  almost  -inky  blackness — black  qualified  by  a 
trace  of  indigo. 

What  change  does  the  home  examination  here 
F'iveal  ?  In  passing  to  indigo,  the  water  becomes  sud- 
denly augmented  in  purity,  the  suspended  matter 
becoming  suddenly  less.  Off  Tarifa,  the  deep  indigo 
disappears,  and  the  sea  is  undecided  in  colour.  Accom- 
panying this  change,  we  have  a  rise  in  the  quantity  of 
suspended  matter.  Beyond  Tarifa,  we  change  to  cobalt- 
blue,  the  suspended  matter  falling  at  the  same  time  in 
quantity.  This  water  is  distinctly  purer  than  the 
green.  We  approach  Cadiz,  and  at  twelve  miles  from 
the  city  get  into  yellow-green  water  ;  this  the  London 
examination  shows  to  be  thick  with  suspended  matter. 
The  same  is  true  of  Cadiz  harbour,  and  also  of  a  point 
fourteen  miles  from  Cadiz  in  the  homeward  direction. 
Here  there  is  a  sudden  change  from  yellow-green  to  a 
bright  emerald-green,  and  accompanying  the  change 
a  sudden  fall  in  the  quantity  of  suspended  matter. 
Between  Cape  St.  Mary  and  Cape  St.  Vincent  the 
water  changes  to  the  deepest  indigo,  a  finrther  diminution 
of  the  suspended  matter  being  the  concomitant  pheno- 
menon. 

We  now  reach  the  remarkable  group  of  rocks  called 
the  Burlings,  and  find  the  water  between  the  shore  and 
the  rocks  a  strong  green  ;  the  home  examination  shows 
it  to  be  thick  with  fine  matter.  Fifteen  or  twenty  miles 
bej^ond  the  Burlings  we  come  again  into  indigo  water, 
from  which  the  suspended  matter  has  in  great  part  dis- 
appeared. Off  Cape  Finisterre,  about  the  place  where 
the  '  Captain '  went  down,  the  water  becomes  green,  and 
the  home  examination  pronounces  it  to  be  thicker.  Then 
we  enter  the  Bay  of  Biscay,  where  the  indigo  resumes 
its  power,  and  where  the  home  examination  shows  the 


170  FRAGMENTS  OF  SCIENCE. 

greatly  augmented  purity  of  the  water.  A  second 
specimen  of  water,  taken  from  the  Bay  of  Biscay,  held 
in  suspension  fine  particles  of  a  peculiar  kind  ;  the  size 
of  them  was  such  as  to  render  the  water  richly 
iridescent.  It  showed  itself  green,  blue,  or  salmon- 
coloured,  according  to  the  direction  of  the  line  of  vision. 
Finally,  we  come  to  our  last  two  bottles,  the  one  taken 
opposite  St.  Catherine's  lighthouse,  in  the  Isle  of  Wight, 
the  other  at  Spithead.  The  sea  at  both  these  places 
was  green,  and  both  specimens,  as  might  be  expected, 
were  pronounced  by  the  home  examination  to  be  thick 
with  suspended  matter. 

Two  distinct  series  of  observations  are  here  referred 
to — the  one  consisting  of  direct  observations  of  the 
colour  of  the  sea,  conducted  during  the  voyage  from 
Gibraltar  to  Portsmouth  :  the  other  carried  out  in  the 
laboratory  of  the  Royal  Institution.  And  here  it  is  to 
be  noted  that  in  the  home  examination  I  never  knew 
what  water  was  placed  in  my  hands.  The  labels,  with 
the  names  of  the  localities  written  upon  them,  had  been 
tied  up,  all  information  regarding  the  source  of  the 
water  being  thus  held  back.  The  bottles  were  simply 
numbered,  and  not  till  all  of  them  had  been  examined, 
and  described,  were  the  labels  opened,  and  the  locality 
and  sea-colour  corresponding  to  the  various  specimens 
ascertained.  The  home  observations,  therefore,  must 
have  been  perfectly  unbiassed,  and  they  clearly 
establish  the  association  of  the  green  colour  with  fine 
suspended  matter,  and  of  the  ultramarine  colour,  and 
more  especially  of  the  black-indigo  hue  of  the  Atlantic, 
with  the  comparative  absence  of  such  matter. 

So  much  for  mere  observation ;  but  what  is  the 
cause  of  the  dark  hue  of  the  deep  ocean  ?  ^     A  prelimi- 

*  A  note,  written  to  me  on  October  22,  by  my  friend ,  Canon 
Kingsley,  contains  the  following  reference  to  this  point :  '  I  have 


VOYAGE  TO  ALGEEIA.  171 

nary  remark  or  two  will  clear  our  way  towards  an  ex- 
planation. Colour  resides  in  white  light,  appearing 
when  any  constituent  of  the  white  light  is  with- 
drawn. The  hue  of  a  purple  liquid,  for  example,  la 
immediately  accounted  for  by  its  action  on  a  spectrum. 
It  cuts  out  the  yellow  and  green,  and  allows  the  red 
and  blue  to  pass  through.  The  blending  of  these  two 
colours  produces  the  purple.  But  while  such  a  liquid 
attacks  with  special  energy  the  yellow  and  green,  it 
enfeebles  the  whole  spectrum.  By  increasing  the  thick- 
ness of  the  stratum  we  may  absorb  the  whole  of  the 
light.  The  colour  of  a  blue  liquid  is  similarly  accounted 
for.  It  first  extinguishes  the  red  ;  then,  as  the  thick- 
ness augments,  it  attacks  the  orange,  yeUow,  and  green 
in  succession ;  the  blue  alone  finally  remaining.  But 
even  it  might  be  extinguished  by  a  sufficient  depth  of 
the  liquid. 

And  now  we  are  prepared  for  a  brief,  but  tolerably 
complete,  statement  of  that  action  of  sea-water  upon 
light,  to  which  it  owes  its  darkness.  The  spectrum 
embraces  three  classes  of  rays — the  thermal,  the  visual, 
and  the  chemical.  These  divisions  overlap  each  other ; 
the  thermal  rays  are  in  part  visual,  the  visual  rays  in 
part  chemical,  and  vice  versa.  The  vast  body  of  thermal 
rays  lie  beyond  the  red,  being  invisible.  These  rays  are 
attacked  with  exceeding  energy  by  water.  They  are 
absorbed  close  to  the  surface  of  the  sea,  and  are  the 
great  agents  in  evaporation.  At  the  same  time  the 
whole  spectrum  suflfers  enfeeblement ;  water  attacks  all 
its  rays,  but  with  different  degrees  of  energy.     Of  the 

never  seen  the  Late  of  Geneva,  but  I  thought  of  the  brilliant 
dazzling  dark  blue  of  the  mid- Atlantic  under  the  sunlight,  and  it^a 
black-blue  iinder  cloud,  both  so  solid  that  one  might  leap  off  the 
Bponson  on  to  it  without  fear ;  this  was  to  me  the  most  wonderful 
thing  which  I  saw  on  my  voyages  to  and  from  the  West  Indies.' 


112  FRAGMENTS  OF  SCIENCE. 

visual  rays,  the  red  are  first  extinguished.  As  the  solai 
beam  plunges  deeper  into  the  sea,  orange  follows  red, 
yellow  follows  orange,  green  follows  yellow,  and  the 
various  shades  of  blue,  where  the  water  is  deep  enough, 
follow  green.  Absolute  extinction  of  the  solar  beam 
would  be  the  consequence  if  the  water  were  deep  and 
uniform.  If  it  contained  no  suspended  matter,  such 
water  would  be  as  black  as.  ink.  A  reflected  glimmer  of 
ordinary  light  would  reach  us  from  its  surface,  as  it 
would  from  the  surface  of  actual  ink;  but  no  light, 
hence  no  colour,  would  reach  us  from  the  body  of  the 
water. 

In  very  clear  and  deep  sea-water  this  condition  is 
approximately  fulfilled,  and  hence  the  extraordinary 
darkness  of  such  water.  The  indigo,  already  referred 
to,  is,  I  believe,  to  be  ascribed  in  part  to  the  suspended 
matter,  which  is  never  absent,  even  in  the  purest  natural 
water ;  and  in  part  to  the  slight  reflection  of  the  light 
from  the  limiting  surfaces  of  strata  of  different  densi- 
ties. A  modicum  of  light  is  thus  thrown  back  to  the 
eye,  before  the  depth  necessary  to  absolute  extinction 
has  been  attained.  An  effect  precisely  similar  occurs 
under  the  moraines  of  glaciers.  The  ice  here  is  ex- 
ceptionally compact,  and,  owing  to  the  absence  of  the 
internal  scattering  common  in  bubbled  ice,  the  light 
plunges  into  the  mass,  where  it  is  extinguished,  the 
perfectly  clear  ice  presenting  an  appearance  of  pitchy 
blackness.* 

The  green  colour  of  the  sea  has  now  to  be  accounted 
for;  and  here,  again,  let  us  fall  back  upon  the  sure 
basis  of  experiment.  A  strong  white  dinner-plate  had 
a  lead  weight  seciurely  fastened  to  it.  Fifty  or  sixty 
yards  of  strong  hempen  line  were  attached  to  the  plate. 

'  I  leaxn  from  a  correspondent  that  certain  Welsh  tarns,  which 
are  reputed  bottomless,  have  this  inky  hue. 


VOYAaE  TO  ALGEKIA.  173 

My  assistant,  Thorogood,  occupied  a  boat,  fastened  as 
usual  to  the  davits  of  the  '  Urgent,'  while  I  occupied  a 
second  boat  nearer  the  stern  of  the  ship.  He  cast  the 
plate  as  a  mariner  heaves  the  lead,  and  by  the  time  it 
reached  me  it  had  sunk  a  considerable  depth  in  the 
water.  In  all  cases  the  hue  of  this  plate  was  green. 
Even  when  the  sea  was  of  the  darkest  indigo,  the  green 
was  vivid  and  pronounced.  I  could  notice  the  gradual 
deepening  of  the  colour  as  the  plate  sank,  but  at  its 
greatest  depth,  even  in  indigo  water,  the  colour  was  still 
a  blue-green.^ 

Other  observations  confirmed  this  one.  The 
'  Urgent '  is  a  screw  steamer,  and  right  over  the  blades 
of  the  screw  was  an  orifice  called  the  screw-well,  through 
which  one  could  look  from  the  poop  down  upon  the 
screw.  The  surface-glimmer,  which  so  pesters  the  eye, 
was  here  in  a  great  measure  removed.  Midway  down, 
a  plank  crossed  the  screw-well  from  side  to  side ;  on 
this  I  placed  myself  and  observed  the  action  of  the 
screw  underneath.  The  eye  was  renderigd  sensitive  by 
the  moderation  of  the  light ;  and,  to  remove  still  further 
all  disturbing  causes,  Lieutenant  Walton  had  a  sail  and 
tarpaulin  thrown  over  the  mouth  of  the  well.  Under- 
neath this  I  perched  myself  on  the  plank  and  watched 
the  screw.  In  an  indigo  sea  the  play  of  colour  was 
indescribably  beautiful,  and  the  contrast  between  the 
water,  which  had  the  screw-blades,  and  that  which  had 
the  bottom  of  the  ocean,  as  a  background,  was  extra- 
ordinary. The  one  was  of  the  most  brilliant  green,  the 
other  of  the  deepest  ultramarine.  The  surface  of  the 
water  above  the  screw-blade  was  always  ruffled.  Liquid 
lenses  were  thus  formed,  by  which  the  coloured  light 
was  withdrawn  from  some  places  and  concentrated  upon 

'  In  no  case,  of  course,  is  the  green  pure,  but  a  mixture  of  green 
axtd  blue. 


174  FRAGMENTS  OF  SCIENCE. 

others,  the  water  flashing  with  metallic  lustre.  The 
screw-blades  in  this  case  played  the  part  of  the  dinner- 
plate  in  the  former  case,  and  there  were  other  instances 
of  a  similar  kind.  The  white  bellies  of  porpoises 
showed  the  green  hue,  varying  in  intensity  as  the 
creatures  swung  to  and  fro  between  the  surface  and 
the  deeper  water.  Foam,  at  a  certain  depth  below  the 
surface,  was  also  green.  In  a  rough  sea  the  light  which 
penetrated  the  summit  of  a  wave  sometimes  reached  the 
eye,  a  beautiful  green  cap  being  thus  placed  upon  the 
wave,  even  in  indigo  water. 

But  how  is  this  colour  to  be  connected  with  the  sus- 
pended particles  ?  Thus.  Take  the  dinner-plate  which 
showed  so  brilliant  a  green  when  thrown  into  indigo 
water.  Suppose  it  to  diminish  in  size,  until  it  reaches 
an  almost  microscopic  magnitude.  It  would  still  behave 
substantially  as  the  larger  plate,  sending  to  the  eye  its 
modicum  of  green  light.  If  the  plate,  instead  of  being 
a  large  coherent  mass,  were  ground  to  a  powder  suffi- 
ciently fine,  and  in  this  condition  diffused  through  the 
clear  sea- water,  it  would  also  send  green  light  to  the  eye. 
In  fact,  the  suspended  particles  which  the  home  exami- 
nation reveals,  act  in  all  essential  particulars  like  the 
plate,  or  like  the  screw-blades,  or  like  the  foam,  or  like 
the  bellies  of  the  porpoises.  Thus  I  think  the  green- 
ness of  the  sea  is  physically  connected  with  the  matter 
which  it  holds  in  suspension. 

We  reached  Portsmouth  on  January  5, 1871.  Then 
ended  a  voyage  which,  though  its  main  object  was  not 
realised,  has  left  behind  it  pleasant  memories,  both  of 
the  aspects  of  nature  and  the  kindliness  of  men. 


VII. 

NIAQARA."^ 

ris  one  of  the  disadvantages  of  reading  books  about 
natural  scenery  that  they  fill  the  mind  with  pictures, 
often  exaggerated,  often  distorted,  often  blurred,  and, 
even  when  well  drawn,  injurious  to  the  freshness  of 
first  impressions.  Such  has  been  the  fate  of  most  of  us 
with  regard  to  the  Falls  of  Niagara.  There  was  little 
accuracy  in  the  estimates  of  the  first  observers  of  the 
cataract.  Startled  by  an  exhibition  of  power  so  novel 
and  so  grand,  emotion  leaped  beyond  the  control  of  the 
judgment,  and  gave  currency  to  notions  which  have 
often  led  to  disappointment. 

A  record  of  a  voyage  in  1 535  by  a  French  mariner 
named  Jacques  Cartier,  contains,  it  is  said,  the  first 
printed  allusion  to  Niagara.  In  1603  the  first  map  of 
the  district  was  constructed  by  a  Frenchman  named 
Champlain.  In  1648  the  Jesuit  Kageneau,  in  a  letter 
to  his  superior  at  Paris,  mentions  Niagara  as  '  a  cataract 
of  frightful  height.' 2  In  the  winter  of  1678  and  1679 
the  cataract  was  visited  by  Father  Hennepin,  and 
described  in  a  book  dedicated  '  to  the  King  of  Great 
Britain.'     He  gives  a  drawing  of  the  waterfall,  which 

*  A  Discourse  delivered  at  the  Koyal  Institution  of  Great 
Britain,  April  4,  1873. 

*  From  an  interesting  little  book  presented  to  me  at  Brooklyn 
by  its  author,  Mr.  Holly,  some  of  these  data  are  derived ;  Hennepin, 
Kalm,  Bakewell,  Lyell,  Hall,  and  others  I  have  myself  consulted. 


176  FEAGMENTS  OF  SCIENCE. 

shows  that  serious  changes  have  taken  place  since  his 
time.  He  describes  it  as  'a  s^reat  and  prodigious 
cadence  of  water,  to  which  the  universe  does  not  otfer  a 
parallel.'  The  height  of  the  fall,  according  to  Hennepin, 
was  more  than  600  feet.  '  The  waters,'  he  says,  '  which 
fall  from  this  great  precipice  do  foam  and  boil  in  the 
most  astonishing  manner,  making  a  noise  more  terrible 
than  that  of  thunder.  When  the  wind  blows  to  the 
south  its  frightful  roaring  may  be  heard  for  more  than 
fifteen  leagues.'  The  Baron  la  Hontan,  who  visited 
Niagara  in  1687,  makes  the  height  800  feet.  In  1721 
Charlevois,  in  a  letter  to  Madame  de  Maintenon,  after 
referring  to  the  exaggerations  of  his  predecessors,  thus 
states  the  result  of  his  own  observations  :  *  For  my  part, 
after  examining  it  on  all  sides,  I  am  inclined  to  think 
that  we  cannot  allow  it  less  than  140  or  150  feet,' — a 
remarkably  close  estimate.  At  that  time,  viz.  a  hundred 
and  fifty  years  ago,  it  had  the  shape  of  a  horseshoe,  and 
reasons  will  subsequently  be  given  for  holding  that  this 
has  been  always  the  form  of  the  cataract,  from  its  origin 
to  its  present  site. 

As  regards  the  noise  of  the  fall,  Charlevois  declares 
the  accounts  of  his  predecessors,  which,  I  may  say,  are 
repeated  to  the  present  hour,  to  be  altogether  extrava- 
gant. He  is  perfectly  right.  The  thunders  of  Niagara 
are  formidable  enough  to  those  who  really  seek  them 
at  the  base  of  the  Horseshoe  Fall;  but  on  the  banks  of 
the  river,  and  particularly  above  the  fall,  its  silence, 
rather  than  its  noise,  is  surprising.  This  arises,  in 
part,  from  the  lack  of  resonance;  the  surrounding 
country  being  flat,  and  therefore  furnishing  no  echoing 
surfaces  to  reinforce  the  shock  of  the  water.  The 
resonance  from  the  surrounding  rocks  causes  the  Swiss 
Reuss  at  the  Devil's  Bridge,  when  full,  to  thunder  more 
loudly  than  the  Niagara. 


NIAGARA.  177 

On  Friday,  November  1,  1872,  just  before  reaching 
the  village  of  Niagara  Falls,  I  caught,  from  the  railway 
train,  my  first  glimpse  of  the  smoke  of  the  cataract. 
Immediately  after  my  arrival  I  went  with  a  friend  to 
the  northern  end  of  the  American  Fall.  It  may  be 
that  my  mood  at  the  time  toned  down  the  impression 
produced  by  the  first  aspect  of  this  grand  cascade ;  but 
I  felt  nothing  like  disappointment,  knowing,  from  old 
experience,  that  time  and  close  acquaintanceship,  the 
gradual  interweaving  of  mind  and  nature,  must  power- 
fully influence  my  final  estimate  of  the  scene.  After 
dinner  we  crossed  to  Goat  Island,  and,  turning  to  the 
right,  reached  the  southern  end  of  the  American  Fall. 
The  river  is  here  studded  with  small  islands.  Crossing 
a  wooden  bridge  to  Luna  Island,  and  clasping  a  tree 
w'hich  grows  near  its  edge,  I  looked  long  at  the  cataract, 
which  here  shoots  down  the  precipice  like  an  avalanche 
of  foam.  It  grew  in  power  and  beauty.  The  channel 
spanned  by  the  wooden  bridge  was  deep,  and  the  river 
there  doubled  over  the  edge  of  the  precipice,  like  the 
swell  of  a  muscle,  unbroken.  The  ledge  here  over- 
hangs, the  water  being  poured  out  far  beyond  the  base 
of  the  precipice.  A  space,  called  the  Cave  of  the 
Winds,  is  thus  enclosed  between  the  wall  of  rock  and 
the  falling  water. 

Goat  Island  ends  in  a  sheer  dry  precipice,  which 
connects  the  American  and  Horseshoe  Falls.  Midway 
between  both  is  a  wooden  hut,  the  residence  of  the  guide 
to  the  Cave  of  the  Winds,  and  from  the  hut  a  winding 
staircase,  called  Biddle's  Stair,  descends  to  the  base  of 
the  precipice.  On  the  evening  of  my  arrival  I  went 
down  this  stair,  and  wandered  along  the  bottom  of  the 
cliff.  One  well-known  factor  in  the  formation  and 
retreat  of  the  cataract  w^as  immediately  observed.  A 
thick  layer  of  limestone  formed  the  upper  portion  of 


178  FBAGMENTS  OF  SCIENCE. 

the  cliff.  This  rested  upon  a  bed  of  soft  shale,  which 
extended  round  the  base  of  the  cataract.  The  violent 
recoil  of  the  water  against  this  yielding  substance 
crumbles  it  away,  undermining  the  ledge  above,  which, 
unsupported,  eventually  breaks  off,  and  produces  the 
observed  recession. 

At  the  southern  extremity  of  the  Horseshoe  is  a 
promontory,  formed  by  the  doubling  back  of  the  gorge 
excavated  by  the  cataract,  and  into  which  it  plunges. 
On  the  promontory  stands  a  stone  building,  called  the 
Terrapin  Tower,  the  door  of  which  had  been  nailed  up 
because  of  the  decay  of  the  staircase  within  it.  Through 
the  kindness  of  Mr.  Townsend,  the  superintendent  of 
Groat  Island,  the  door  was  opened  for  me.  From  this 
tower,  at  all  hours  of  the  day,  and  at  some  hours  of  the 
night,  I  watched  and  listened  to  the  Horseshoe  Fall. 
The  river  here  is  evidently  much  deeper  than  the 
American  branch ;  and  instead  of  bursting  into  foam 
where  it  quits  the  ledge,  it  bends  solidly  over,  and  falls 
in  a  continuous  layer  of  the  most  vivid  green.  The 
tint  is  not  uniform ;  long  stripes  of  deeper  hue  alter- 
nating with  bands  of  brighter  colour.  Close  to  the 
ledge  over  which  the  water  rolls,  foam  is  generated,  the 
b'ght  falling  upon  which,  and  flashing  back  from  it,  is 
sifted  in  its  passage  to  and  fro,  and  changed  from  white 
to  emerald-green.  Heaps  of  superficial  foam  are  also 
formed  at  intervals  along  the  ledge,  and  are  imme- 
diately drawn  into  long  white  striae.^  Lower  down, 
the  surface,  shaken  by  the  reaction  from  below,  in- 
cessantly rustles  into  whiteness.  The  descent  finally 
resolves  itself  into  a  rhythm,  the  water  reaching  the 
bottom   of  the  fall  in  periodic   gushes.      Nor  is  the 

*  The  direction  of  the  wind  with  reference  to  the  course  of  a 
ship  may  be  inferred  with  accuracy  from  the  foam-streaks  on  the 
Burface  of  the  sea. 


NIAGAEA.  179 

epray  uniformly  diffused  through  the  air,  but  is  wafted 
through  it  in  successive  veils  of  gaiize-like  texture. 
From  all  this  it  is  evident  that  beauty  is  not  absent 
from  the  Horseshoe  Fall,  but  majesty  is  its  chief 
attribute.  The  plunge  of  the  water  is  not  wild,  but 
deliberate,  vast,  and  fascinating.  From  the  Terrapin 
Tower,  the  adjacent  arm  of  the  Horseshoe  is  seen 
projected  against  the  opposite  one,  midway  down ;  to 
the  imagination,  therefore,  is  left  the  picturing  of  the 
gulf  into  which  the  cataract  plunges. 

The  delight  which  natural  scenery  produces  in  some 
minds  is  difficult  to  explain,  and  the  conduct  which  it 
prompts  can  hardly  be  fairly  criticised  by  those  who 
have  never  experienced  it.  It  seems  to  me  a  deduction 
from  the  completeness  of  the  celebrated  Thomas  Young, 
that  he  was  unable  to  appreciate  natural  scenery.  '  He 
had  really,'  says  Dean  Peacock, '  no  taste  for  life  in  the 
country  ;  he  was  one  of  those  who  thought  that  no  one 
who  was  able  to  live  in  London  would  be  content  to 
live  elsewhere.'  Well,  Dr.  Young,  like  Dr.  Johnson, 
had  a  right  to  his  delights ;  but  I  can  understand  a 
hesitation  to  accept  them,  high  as  they  were,  to  the 
exclusion  of 

That  o'erflowing  joy  which  Nature  yields 
To  her  true  lovers. 

To  all  who  are  of  this  mind,  the  strengthening  of 
desire  on  my  part  to  see  and  know  Niagara  Falls,  as 
far  as  it  is  possible  for  them  to  be  seen  and  known,  will 
be  intelligible. 

On  the  first  evening  of  my  visit,  I  met,  at  the  head 
of  Biddle's  Stair,  the  guide  to  the  Cave  of  the  Winds. 
He  was  in  the  prime  of  manhood — large,  well  built, 
firm  and  pleasant  in  mouth  and  eye.  My  interest  in 
the  scene  stirred  up  his,  and  made  him  communicative. 


180  FKAGMENTS  OF  SCIENCE. 

Turning  to  a  photograph,  he  described,  by  reference  to 
it,  a  feat  which  he  had  accomplished  some  time  pre- 
viously, and  which  had  brought  him  almost  under  the 
green  water  of  the  Horseshoe  Fall.  '  Can  you  lead  me 
there  to-morrow  ?  '  I  asked.  He  eyed  me  enquiringly, 
weighing,  perhaps,  the  chances  of  a  man  of  light  build, 
and  with  grey  in  his  whiskers,  in  such  an  undertaking. 
*  I  wish,'  I  added,  '  to  see  as  much  of  the  fall  as  can  be 
seen,  and  where  you  lead  I  will  endeavour  to  follow.' 
His  scrutiny  relaxed  into  a  smile,  and  he  said,  '  Very 
well ;  I  shall  be  ready  for  you  to-morrow.' 

On  the  morrow,  accordingly,  I  came.  In  the  hut 
at  the  head  of  Biddle's  Stair  I  stripped  wholly,  and 
re-dressed  according  to  instructions, — drawing  on  two 
pairs  of  woollen  pantaloons,  three  woollen  jackets,  two 
pairs  of  socks,  and  a  pair  of  felt  shoes.  Even  if  wet, 
my  guide  assured  me  that  the  clothes  would  keep  me 
from  being  chilled  ;  and  he  was  right.  A  suit  and 
hood  of  yellow  oilcloth  covered  all.  Most  laudable  pre- 
cautions were  taken  by  the  young  assistant  who  helped 
to  dress  me  to  keep  the  water  out;  but  his  devices 
broke  down  immediately  when  severely  tested. 

We  descended  the  stair ;  the  handle  of  a  pitchfork 
doing,  in  my  case,  the  duty  of  an  alpenstock.  At  the 
bottom,  the  guide  enquired  whether  we  should  go  first 
to  the  Cave  of  the  Winds,  or  to  the  Horseshoe,  remark- 
ing that  the  latter  would  try  us  most.  I  decided  on 
getting  the  roughest  done  first,  and  he  turned  to  the 
left  over  the  stones.  They  were  sharp  and  trying.  The 
base  of  the  first  portion  of  the  cataract  is  covered  vdth 
huge  boulders,  obviously  the  ruins  of  the  limestone 
ledge  above.  The  water  does  not  distribute  itself  uni- 
formly among  these,  but  seeks  out  channels  through 
which  it  pours  torrentially.  We  passed  some  of 
these  with  wetted  feet,   but  without   difficulty.      At 


NIAGARA.  181 

length  we  came  to  the  side  of  a  more  formidable 
current.  My  guide  walked  along  its  edge  until  he 
reached  its  least  tui-bulent  portion.  Halting,  he  said, 
'  This  is  our  greatest  difficulty ;  if  we  can  cross  here, 
we  shall  get  far  towards  the  Horseshoe.' 

He  waded  in.  It  evidently  required  all  his  strength 
to  steady  him.  The  water  rose  above  his  loins,  and  it 
foamed  still  higher.  He  had  to  search  for  footing, 
amid  unseen  boulders,  against  which  the  torrent  rose 
violently.  He  struggled  and  swayed,  but  he  struggled 
successfully,  and  finally  reached  the  shallower  water  at 
the  other  side.  Stretching  out  his  arm,  he  said  to  me, 
'  Now  come  on.'  I  looked  down  the  torrent,  as  it 
rushed  to  the  river  below,  which  was  seething  with  the 
tumult  of  the  cataract.  De  Saussure  recommended 
the  inspection  of  Alpine  dangers,  with  the  view  of 
making  them  familiar  to  the  eye  before  they  are  en- 
countered ;  and  it  is  a  wholesome  custom  in  places  of 
difficulty  to  put  the  possibility  of  an  accident  clearly 
before  the  mind,  and  to  decide  beforehand  what  ought 
to  be  done  should  the  accident  occur.  Thus  wound  up 
in  the  present  instance,  I  entered  the  water.  Even 
where  it  was  not  more  than  knee-deep,  its  power  was 
manifest.  As  it  rose  around  me,  I  sought  to  split  the 
torrent  by  presenting  a  side  to  it ;  but  the  insecurity 
of  the  footing  enabled  it  to  grasp  my  loins,  twist  me 
fairly  round,  and  bring  its  impetus  to  bear  upon  my 
back.  Further  struggle  was  impossible ;  and  feeling 
my  balance  hopelessly  gone,  I  turned,  flung  myself 
towards  the  bank  just  quitted,  and  was  instantly,  as 
expected,  swept  into  shallower  water. 

The  oilcloth  covering  was  a  great  incumbrance ;  it 
had  been  made  for  a  much  stouter  man,  and,  standing 
upright  after  my  submersion,  my  legs  occupied  the 
centre  of  two  bags  of  water.     My  guide  exhorted  me  to 


182  FRAGMENTS  OF  SCIENCE. 

try  again.  Prudence  was  at  my  elbow,  whispering 
dissuasion;  but,  taking  everything  into  account,  it 
appeared  more  immoral  to  retreat  than  to  proceed. 
Instructed  by  the  first  misadventure,  I  once  more 
entered  the  stream.  Had  the  alpenstock  been  of  iron 
it  might  have  helped  me ;  but,  as  it  was,  the  tendency 
of  the  water  to  sweep  it  out  of  my  hands  rendered  it 
worse  than  useless.  I,  however,  clung  to  it  by  habit. 
Again  the  torrent  rose,  and  again  I  wavered ;  but,  by 
keeping  the  left  hip  well  against  it,  I  remained  upright, 
and  at  length  grasped  the  hand  of  my  leader  at  the 
other  side.  He  laughed  pleasantly.  The  first  victory 
was  gained,  and  he  enjoyed  it.  '  No  traveller,'  he  said, 
'  was  ever  here  before.'  Soon  afterwards,  by  trusting 
to  a  piece  of  drift-wood  which  seemed  firm,  I  was  again 
taken  ofi"  my  feet,  but  was  immediately  caught  by  a 
protruding  rock. 

We  clambered  over  the  boulders  towards  the  thickest 
spray,  which  soon  became  so  weighty  as  to  cause  us  to 
stagger  under  its  shock.  For  the  most  part  nothing 
could  be  seen ;  we  were  in  the  midst  of  bewildering 
tumult,  lashed  by  the  water,  which  sounded  at  times 
like  the  cracking  of  innumerable  whips.  Underneath 
this  was  the  deep  resonant  roar  of  the  cataract.  I 
tried  to  shield  my  eyes  with  my  hands,  and  look  up- 
wards ;  but  the  defence  was  useless.  The  guide  con- 
tinued Uj-  move  on,  but  at  a  certain  place  he  halted, 
desiring  me  to  take  shelter  in  his  lee,  and  observe 
the  cataract.  The  spray  did  not  come  so  much  from 
the  upper  ledge,  as  from  the  rebound  of  the  shattered 
water  when  it  struck  the  bottom.  Hence  the  eyes 
could  be  protected  from  the  blinding  shock  of  the 
spray,  while  the  line  of  vision  to  the  upper  ledges 
remained  to  some  extent  clear.  On  looking  upwards 
over  the  guide's  shoulder  I  could  see  the  water  bending 


NIAGARA.  183 

over  the  ledge,  while  the  Terrapin  Tower  loomed  fitfully- 
through  the  intermittent  spray-gusts.  We  were  right 
under  the  tower.  A  little  farther  on  the  cataract,  after 
its  first  pluDge,  hit  a  protuberance  some  way  down, 
and  flew  from  it  in  a  prodigious  burst  of  spray  ;  through 
this  we  staggered.  We  rounded  the  promontory  on 
which  the  Terrapin  Tower  stands,  and  moved,  amid 
the  wildest  commotion,  along  the  arm  of  the  Horse- 
shoe, until  the  boulders  failed  us,  and  the  cataract  fell 
into  the  profound  gorge  of  the  Niagara  Eiver. 

Here  the  guide  sheltered  me  again,  and  desired  me 
to  look  up;  I  did  so,  and  could  see,  as  before,  the 
green  gleam  of  the  mighty  curve  sweeping  over  the 
upper  ledge,  and  the  fitful  plunge  of  the  water,  as 
the  spray  between  us  and  it  alternately  gathered  and 
disappeared.  An  eminent  friend  of  mine  often  speaks 
of  the  mistake  of  those  physicians  who  regard  man's 
ailments  as  purely  chemical,  to  be  met  by  chemical 
remedies^  only.  He  contends  for  the  psychological 
element  of  cure.  By  agreeable  emotions,  he  says, 
nervous  currents  are  liberated  which  stimulate  blood, 
brain,  and  viscera.  The  influence  rained  from  ladies' 
eyes  CDables  my  friend  to  thrive  on  dishes  which  would 
kill  him  if  eaten  alone.  A  sanative  effect  of  the  same 
order  I  experienced  amid  the  spray  and  thunder  of 
Niagara.  Quickened  by  the  emotions  there  aroused, 
the  blood  sped  exultingly  through  the  arteries,  abolish- 
ing introspection,  clearing  the  heart  of  all  bitterness, 
and  enabling  one  to  think  with  tolerance,  if  not  with 
tenderness,  on  the  most  relentless  and  unreasonable  foe. 
Apart  from  its  scientific  value,  and  purely  as  a  moral 
agent,  the  play  was  worth  the  candle.  My  companion 
knew  no  more  of  me  than  that  I  enjoyed  the  wildness 
of  the  scene  ;  but  as  I  bent  in  the  shelter  of  his  large 
frame  he  said,  '  I  should  like  to  see  you  attempting  to 
.13 


184  FRAGMENTS  OF  SCIENCE. 

describe  all  this.'     He  rightly  thought  it  indescribable. 
The  name  of  this  gallant  fellow  was  Thomas  Conroy. 

We  returned,  clambering  at  intervals  up  and  down, 
so  as  to  catch  glimpses  of  the  most  impressive  portions 
of  the  cataract.  We  passed  under  ledges  formed  by 
tabular  masses  of  limestone,  and  through  some  curious 
openings  formed  by  the  falling  together  of  the  summits 
of  the  rocks.  At  length  we  found  ourselves  beside  our 
enemy  of  the  morning.  Conroy  halted  for  a  minute  or 
two,  scanning  the  torrent  thoughtfully.  I  said  that,  as 
a  guide,  he  ought  to  have  a  rope  in  such  a  place ;  but 
he  retorted  that,  as  no  traveller  had  ever  thought  of 
coming  there,  he  did  not  see  the  necessity  of  keeping 
a  rope.  He  waded  in.  The  struggle  to  keep  himself 
erect  was  evident  enough ;  he  swayed,  but  recovered 
himself  again  and  again.  At  length  he  slipped,  gave 
way,  did  as  I  had  done,  threw  himself  towards  the 
bank,  and  was  swept  into  the  shallows.  Standing  in 
the  stream  near  its  edge,  he  stretched  his  arm  towards 
me.  I  retained  the  pitchfork  handle,  for  it  had  been 
useful  among  the  boulders.  By  wading  some  way  in, 
the  staff  could  be  made  to  reach  him,  and  I  proposed 
his  seizing  it.  '  If  you  are  sure,'  he  replied,  '  that,  in 
case  of  giving  way,  you  can  maintain  your  grasp,  then 
I  will  certainly  hold  you.'  Remarking  that  he  might 
count  on  this,  I  waded  in,  and  stretched  the  staff  to 
my  companion.  It  was  firmly  grasped  by  both  of  us. 
Thus  helped,  though  its  onset  was  strong,  I  moved 
safely  across  the  torrent.  All  danger  ended  here.  We 
afterwards  roamed  sociably  among  the  torrents  and 
boulders  below  the  Cave  of  the  Winds.  The  rocks 
were  covered  with  organic  slime,  which  could  not  have 
been  walked  over  with  bare  feet,  but  the  felt  shoes 
effectually  prevented  slipping.  We  reached  the  cave 
and  entered  it,  first  by  a  wooden  way  carried  over  the 


NIAGARA.  185 

boulders,  and  then  along  a  narrow  ledge,  to  the  point 
eaten  deepest  into  the  shale.  When  the  wind  is  from 
the  south,  the  falling  water,  I  am  told,  can  be  seen 
tranquilly  from  this  spot ;  but  when  we  were  there,  a 
blinding  hurricane  of  spray  was  whirled  against  us. 
On  the  evening  of  the  same  day,  I  went  behind  the 
water  on  the  Canada  side,  which,  after  the  experiences 
of  the  morning,  struck  me  as  an  imposture. 

Still  even  this  latter  is  exciting  to  some  nerves. 
Its  effect  upon  himself  is  thus  vividly  described  by 
Mr.  Bakewell,  jun. :  '  On  turning  a  sharp  angle  of  the 
rock,  a  sudden  gust  of  wind  met  us,  coming  from  the 
hollow  between  the  fall  and  the  rock,  which  drove  the 
spray  directly  in  our  faces,  with  such  force  that  in  an 
instant  we  were  wet  through.  When  in  the  midst  of 
this  shower-bath  the  shock  took  away  my  breath :  I 
turned  back  and  scrambled  over  the  loose  stones  to 
escape  the  conflict.  The  guide  soon  followed,  and  told 
me  that  I  had  passed  the  worst  part.  With  that 
assurance  I  made  a  second  attempt ;  but  so  wild  and 
disordered  was  my  imagination  that  when  I  had 
reached  half  way  I  could  bear  it  no  longer.*  * 

To  complete  my  knowledge  I  desired  to  see  the  fall 
from  the  river  below  it,  and  long  negotiations  were 
necessary  to  secure  the  means  of  doing  so.  The  only 
boat  fit  for  the  undertaking  had  been  laid  up  for  the 
winter ;  but  this  difficulty,  through  the  kind  interven- 
tion of  Mr.  Townsend,  was  overcome.  The  main  one 
was  to  secure  oarsmen  sufficiently  strong  and  skilful  to 
urge  the  boat  where  I  wished  it  to  be  taken.  The  son 
of  the  owner  of  the  boat,  a  finely-built  young  fellow, 
but  only  twenty,  and  therefore  not  sufficiently  hardened, 
was  willing  to  go ;  and  up  the  river,  it  was  stated,  there 
lived  another  man  who  could  do  anything  with  the 
•  'Mag.  of  Nat.  Hist.,'  1830,  pp.  121,  122. 


186  FRAGMENTS   OF  SCIENCE. 

boat  whicli  strength  and  daring  could  accomplish.  He 
came.  His  figure  and  expression  of  face  certainly 
indicated  extraordinary  firmness  and  power.  On  Tues- 
day, November  5,  we  started,  each  of  us  being  clad  in 
oilcloth.  The  elder  oarsman  at  once  assumed  a  tone 
of  authority  over  his  companion,  and  struck  imme- 
diately in  amid  the  breakers  below  the  American  Fall. 
He  hugged  the  cross  freshets  instead  of  striking  out 
into  the  smoother  water.  I  asked  him  why  he  did  so, 
and  he  replied  that  they  were  directed  outwards,  not 
downwards.  The  struggle,  however,  to  prevent  the 
bow  of  the  boat  from  being  turned  by  them,  was  often 
very  severe. 

The  spray  was  in  general  blinding,  but  at  times  it 
disappeared  and  yielded  noble  views  of  the  fall.  The 
edge  of  the  cataract  is  crimped  by  indentations  which 
exalt  its  beauty.  Here  and  there,  a  little  below  the 
highest  ledge,  a  secondary  one  juts  out;  the  water 
strikes  it  and  bursts  from  it  in  huge  protuberant  masses 
of  foam  and  spray.  We  passed  Groat  Island,  came  to  the 
Horseshoe,  and  worked  for  a  time  along  its  base,  the 
boulders  over  which  Conroy  and  myself  had  scrambled 
a  few  days  previously  lying  between  us  and  the 
cataract.  A  rock  was  before  us,  concealed  and  revealed 
at  intervals,  as  the  waves  passed  over  it.  Our  leader 
tried  to  get  above  this  rock,  first  on  the  outside  of  it. 
The  water,  however,  was  here  in  violent  motion.  The 
men  struggled  fiercely,  the  older  one  ringing  out  an 
incessant  peal  of  command  and  exhortation  to  the 
younger.  As  we  were  just  clearing  the  rock,  the  bow 
came  obliquely  to  the  surge ;  the  boat  was  turned  sud- 
denly round  and  shot  with  astonishing  rapidity  down 
the  river.  The  men  returned  to  the  charge,  now 
trying  to  get  up  between  the  half-concealed  rock  and 
the  boulders  to  the  left.    But  the  torrent  set  in  strongly 


NIAGARA.  187 

through  this  channel.  The  tugging  was  quick  and 
violent,  but  we  made  little  way.  At  length,  seizing  a 
rope,  the  principal  oarsman  made  a  desperate  attempt 
to  get  upon  one  of  the  boulders,  hoping  to  be  able  to 
drag  the  boat  through  the  channel ;  but  it  bumped  so 
violently  against  the  rock,  that  the  man  flung  himself 
back  and  relinquished  the  attempt. 

We  returned  along  the  base  of  the  American  Fall, 
running  in  and  out  among  the  currents  which  rushed 
from  it  laterally  into  the  river.  Seen  from  below  the 
American  Fall  is  certainly  exquisitely  beautiful,  but  it 
is  a  mere  frill  of  adornment  to  its  nobler  neighbour  the 
Horseshoe.  At  times  we  took  to  the  river,  from  the 
centre  of  which  the  Horseshoe  Fall  appeared  especially 
magnificent.  A  streak  of  cloud  across  the  neck  of 
Mont  Blanc  can  double  its  apparent  height,  so  here 
the  green  summit  of  the  cataract  shining  above  the 
smoke  of  spray  appeared  lifted  to  an  extraordinary 
elevation.  Had  Hennepin  and  La  Hontan  seen  the 
fall  from  this  position,  their  estimates  of  the  height 
would  have  been  perfectly  excusable. 

From  a  point  a  little  way  below  the  American  Fall, 
a  ferry  crosses  the  river,  in  summer,  to  the  Canadian 
side.  Below  the  ferry  is  a  suspension  bridge  for 
carriages  and  foot-passengers,  and  a  mile  or  two  lower 
down  is  the  railway  suspension  bridge.  Between  ferry 
and  bridge  the  river  Niagara  flows  unruffled ;  but  at 
the  suspension  bridge  the  bed  steepens  and  the  river 
quickens  its  motion.  Lower  down  the  gorge  narrows, 
and  the  rapidity  and  turbulence  increase.  At  the  place 
called  the  '  Whirlpool  Kapids  '  I  estimated  the  width  of 
the  river  at  300  feet,  an  estinaate  confirmed  by  the 
dwellers  on  the  spot.  When  it  is  remembered  that  the 
drainage  of  nearly  half  a  continent  is  compressed  into 


188  FRAGMENTS  OF  SCIENCE. 

this  space,  the  impetuosity  of  the  river's  rush  may  be 
imagined.  Had  it  not  been  for  Mr.  Bierstadt,  the 
distinguished  photographer  of  Niagara,  I  should  have 
quitted  the  place  without  seeing  tlieae  rapids;  for 
this,  and  for  his  agreeable  company  to  the  spot,  I  have 
to  thank  him.  From  the  edge  of  the  cliff  above  the 
rapids,  we  descended,  a  little,  I  confess,  to  a  climber's 
disgust,  in  an  '  elevator,'  because  the  effects  are  best 
seen  from  the  water  level. 

Two  kinds  of  motion  are  here  obviously  active,  a 
motion  of  translation  and  a  motion  of  undulation — the 
race  of  the  river  through  its  gorge,  and  the  great  waves 
generated  by  its  collision  with,  and  rebound  from,  the 
obstacles  in  its  way.  In  the  middle  of  the  river  the 
rush  and  tossing  are  most  violent ;  at  all  events,  the 
impetuous  force  of  the  individual  waves  is  here  most 
strikingly  displayed.  Vast  pyramidal  heaps  leap  inces- 
santly from  the  river,  some  of  them  with  such  energy 
as  to  jerk  their  summits  into  the  air,  where  they  hang 
momentarily  suspended  in  crowds  of  liquid  spherules. 
The  sun  shone  for  a  few  minutes.  At  times  the  wind, 
coming  up  the  river,  searched  and  sifted  the  spray,  carry- 
ing away  the  lighter  drops,  and  leaving  the  heavier  ones 
behind.  Wafted  in  the  proper  direction,  rainbows 
appeared  and  disappeared  fitfidly  in  the  lighter  mist. 
In  other  directions  the  common  gleam  of  the  sunshine 
from  the  waves  and  their  shattered  crests  was  exqui- 
sitely beautiful.  The  complexity  of  the  action  was  still 
further  illustrated  by  the  fact,  that  in  some  cases,  as  if 
by  the  exercise  of  a  local  explosive  force,  the  drops  were 
shot  radially  from  a  particular  centre,  forming  around 
it  a  kind  of  halo. 

The  first  impression,  and,  indeed,  the  current  ex- 
planation of  these  rapids  is,  that  the  central  bed  of 
the  river  is  cumbered  with  large  boulders,  and  that  the 


NUGARA.  189 

jostling,  tossing,  and  wild  leaping  of  the  water  there, 
are  due  to  its  impact  against  these  obstacles.  I  doubt 
this  explanation.  At  all  events,  there  is  another  suffi- 
cient reason  to  be  taken  into  account.  Boulders  de- 
rived from  the  adjacent  cliffs  visibly  cumber  the  sides 
of  the  river.  Against  these  the  water  rises  and  sinks 
rhythmically  but  violently,  large  waves  being  thus 
produced.  On  the  generation  of  each  wave,  there  is 
an  immediate  compounding  of  the  wave-motion  with 
the  river-motion.  The  ridges,  which  in  still  water 
would  proceed  in  circular  curves  round  the  centre  of 
disturbance,  cross  the  river  obliquely,  and  the  result  is 
that  at  the  centre  waves  commingle,  which  have  really 
been  generated  at  the  sides.  In  the  first  instance,  we 
had  a  composition  of  wave-motion  with  river-motion ; 
here  we  have  the  coalescence  of  waves  with  waves. 
Where  crest  and  furrow  cross  each  other,  the  motion  is 
annulled ;  where  furrow  and  furrow  cross,  the  river  i?^ 
ploughed  to  a  greater  depth ;  and  where  crest  and  crest 
aid  each  other,  we  have  that  astonishing  leap  of  the 
water  which  breaks  the  cohesion  of  the  crests,  and 
tosses  them  shattered  into  the  air.  From  the  water 
level  the  cause  of  the  action  is  not  so  easily  seen  ;  but 
from  the  summit  of  the  cliff  the  lateral  generation  of 
the  waves,  and  their  propagation  to  the  centre,  are 
perfectly  obvious.  If  this  explanation  be  correct,  the 
phenomena  observed  at  the  Whirlpool  Eapids  form  one 
of  the  grandest  illustrations  of  the  principle  of  inter- 
ference. The  Nile  '  cataract,'  Mr.  Huxley  informs  me, 
offers  more  moderate  examples  of  the  same  action. 

At  some  distance  below  the  Whirlpool  Eapids  we 
have  the  celebrated  whirlpool  itself.  Here  the  river 
makes  a  sudden  bend  to  the  north-east,  forming  nearly 
a  right  angle  with  its  previous  direction.  The  water 
strikes  the  concave  bank  with  great  force,  and  scoops  it 


190  FRAGMENTS  OF  SCIENCE. 

incessantly  away.  A  vast  basin  has  been  thus  formed, 
in  which  the  sweep  of  the  river  prolongs  itself  in 
gyratory  currents.  Bodies  and  trees  which  have  come 
over  the  falls,  are  stated  to  circulate  here  for  days  with- 
out finding  the  outlet.  From  various  points  of  the 
cliffs  above,  this  is  curiously  hidden.  The  rush  of  the 
river  into  the  whirlpool  is  obvious  enough ;  and  though 
you  imagine  the  outlet  must  be  visible,  if  one  existed, 
you  cannot  find  it.  Turning,  however,  round  the  bend 
of  tlie  precipice  to  the  north-east,  the  outlet  comes  into 
view. 

The  Niagara  season  was  over  ;  the  chatter  of  sight- 
seers had  ceased,  and  the  scene  presented  itself  as  one 
of  holy  seclusion  and  beauty.  I  went  down  to  the 
river's  edge,  where  the  weird  loneliness  seemed  to  in- 
crease. The  basin  is  enclosed  by  high  and  almost 
precipitous  banks — covered,  at  the  time,  with  russet 
woods.  A  kind  of  mystery  attaches  itself  to  gyrating 
water,  due  perhaps  to  the  fact  that  we  are  to  some 
extent  ignorant  of  the  direction  of  its  force.  It  is  said 
that  at  certain  points  of  the  whirlpool,  pine-trees  are 
sucked  down,  to  be  ejected  mysteriously  elsewhere. 
The  water  is  of  the  brightest  emerald -green.  The 
gorge  through  which  it  escapes  is  narrow,  and  the 
motion  of  the  river  swift  though  silent.  The  surface  is 
steeply  inclined,  but  it  is  perfectly  unbroken.  There 
are  no  lateral  waves,  no  ripples  with  their  breaking 
bubbles  to  raise  a  murmur ;  while  the  depth  is  here  too 
great  to  allow  the  inequality  of  the  bed  to  ruffle  the 
surface.  Nothing  can  be  more  beautiful  than  this 
sloping  liquid  mirror  formed  by  the  Niagara,  in  sliding 
from  the  whirlpool. 

The  green  colour  is,  I  think,  correctly  accounted  for 
in  the  last  Fragment.  While  crossing  the  Atlantic  in 
1872-73  I  had  frequent  opportunities  of  testing  the  ex- 


NIAGARA.  191 

planation  there  given.  Looked  properly  down  upon,  there 
are  portions  of  the  ocean  to  which  we  should  hardly  ascribe 
a  trace  of  blue  ;  at  the  most,  a  mere  hint  of  indigo  reaches 
the  eye.  The  water,  indeed,  is  practically  black,  and  this 
is  an  indication  both  of  its  depth  and  of  its  freedom  from 
mechanically  suspended  matter.  In  small  thicknesses 
water  is  sensibly  transparent  to  all  kinds  of  light ;  but, 
as  the  thickness  increases,  the  rays  of  low  refiangibility 
are  first  absorbed,  and  after  them  the  other  rays. 
Where,  therefore,  the  water  is  very  deep  and  very  pure, 
all  the  colours  are  absorbed,  and  such  water  ought  to 
appear  black,  as  no  light  is  sent  from  its  interior  to  the 
eye.  The  approximation  of  the  Atlantic  Ocean  to  this 
condition  is  an  indication  of  its  extreme  purity. 

Throw  a  white  pebble  into  such  water ;  as  it  sinks 
it  becomes  greener  and  greener,  and,  before  it  disap- 
pears, it  reaches  a  vivid  blue-green.  Break  sucb  a 
pebble  into  fragments,  each  of  these  will  behave  like 
the  unbroken  mass ;  grind  the  pebble  to  powder,  every 
particle  will  yield  its  modicum  of  green ;  and  if  the 
particles  be  so  fine  as  to  remain  suspended  in  the  water, 
the  scattered  light  will  be  a  uniform  green.  Hence  the 
greenness  of  shoal  water.  You  go  to  bed  with  the  black 
Atlantic  around  you.  You  rise  in  the  morning,  find  it 
a  vivid  green,  and  correctly  infer  that  you  are  crossing 
the  bank  of  Newfoundland.  Such  water  is  found 
charged  with  fine  matter  in  a  state  of  mechanical 
suspension.  The  light  from  the  bottom  may  sometimes 
come  into  play,  but  it  is  not  necessary,  A  storm  can 
render  the  water  muddy,  by  rendering  the  particles  too 
numerous  and  gross.  Such  a  case  occurred  towards  the 
close  of  my  visit  to  Niagara.  There  had  been  rain  and 
storm  in  the  upper  lake-regions,  and  the  quantity  of 
suspended  matter  brought  down  quite  extinguished  the 
fascinating  green  of  the  Horseshoe. 


t 


192  FRA.GMENTS  OF  SCIENCE. 

Nothing  can  be  more  superb  than  the  green  of  the 
Atlantic  waves,  when  the  circumstances  are  favourable 
to  the  exhibition  of  the  colour.  As  long  as  a  wave 
remains  unbroken  no  colour  appears;  but  when  the 
foam  just  doubles  over  the  crest,  like  an  Alpine  snow- 
cornice,  under  the  cornice  we  often  see  a  display  of  the 
most  exquisite  green.  It  is  metallic  in  its  brilliancy. 
But  the  foam  is  necessary  to  its  production.  The  foam 
is  first  illuminated,  and  it  scatters  the  light  in  all  direc- 
tions; the  light  which  passes  through  the  higher  portion 
of  the  wave  alone  reaches  the  eye,  and  gives  to  that 
portion  its  matchless  colour.  The  folding  of  the  wave, 
producing  as  it  does  a  series  of  longitudinal  protuber- 
ances and  furrows  which  act  like  cylindrical  lenses, 
introduces  variations  in  the  intensity  of  the  light,  and 
materially  enhances  its  beauty. 

We  have  now  to  consider  the  genesis  and  proximate 
destiny  of  the  Falls  of  Niagara.  We  may  open  our 
way  to  this  subject  by  a  few  preliminary  remarks  upon 
erosion.  Time  and  intensity  are  the  main  factors  of 
geologic  change,  and  they  are  in  a  certain  sense  conver- 
tible. A  feeble  force  acting  through  long  periods,  and 
an  intense  force  acting  through  short  ones,  may  produce 
approximately  the  same  results.  To  Dr.  Hooker  I  have 
been  indebted  for  some  specimens  of  stones,  the  first 
examples  of  which  were  picked  up  by  Mr.  Hackworth 
on  the  shores  of  Lyell's  Bay,  near  Wellington,  in  New 
Zealand.  They  were  described  by  Mr.  Travers  in  the 
'  Transactions  of  the  New  Zealand  Institute.'  Un- 
acquainted with  their  origin,  you  would  certainly  ascribe 
their  forms  to  human  workmanship.  They  resemble 
knives  and  spear-heads,  being  apparently  chiselled  off 
into  facets,  with  as  much  attention  to  symmetry  as  if  a 
tool,  guided  by  human  intelligence,  had   passed  over 


NIAGAEA.  193 

them.  But  no  human  instrument  has  been  brought  to 
bear  upon  these  stones.  They  have  been  wrought  into 
their  present  shape  by  the  wind-blown  sand  of  LyelPs 
Bay.  Two  winds  are  dominant  here,  and  they  in 
succession  urged  the  sand  against  opposite  sides  of  the 
stone ;  every  little  particle  of  sand  chipped  away  its 
infinitesimal  bit  of  stone,  and  in  the  end  sculptured  these 
singular  forms.' 

The  Sphynx  of  Egypt  is  nearly  covered  up  by  the 
sand  of  the  desert.  The  neck  of  the  Sphynx  is  partly 
cut  across,  not,  as  I  am  assured  by  Mr.  Huxley,  by 
ordinary  weathering,  but  by  the  eroding  action  of  the 
fine  sand  blown  against  it.  In  these  cases  Nature 
furnishes  us  with  hints  which  may  be  taken  advantage 
of  in  art;  and  this  action  of  sand  has  been  recently 
turned  to  extraordinary  account  in  the  United  States. 
When  in  Boston,  I  was  taken  by  my  courteous  and  help- 
ful friend,  Mr.  Josiah  Quincey,  to  see  the  action  of  the 
sand-blast.     A  kind  of  hopper  containing  fine  silicious 

*  *  These  stones,  which  have  a  strong  resemblance  to  works  of 
human  art,  occur  in  great  abundance,  and  of  various  sizes,  from 
half-an-inch  to  several  inches  in  length.  A  large  number  were 
exhibited  showing  the  various  forms,  which  are  those  of  wedges, 
knives,  arrow-heads,  &c.,  and  all  with  sharp  cutting  edges. 

*Mr.  Travers  explained  that,  notwithstanding  their  artificial 
appearance,  these  stones  were  formed  by  the  cutting  action  of  the 
wind-driven  sand,  as  it  passed  to  and  fro  over  an  exposed  boulder- 
bank.  He  gave  a  minute  account  of  the  manner  in  which  the 
varieties  of  form  are  produced,  and  referred  to  the  effect  which 
the  erosive  action  thus  indicated  would  have  on  railway  and  other 
works  executed  on  sandy  tracts. 

*  Dr.  Hector  stated  that  although,  as  a  group,  the  specimens  on 
the  table  could  not  well  be  mistaken  for  artificial  productions,  still 
the  forms  are  so  peculiar,  and  the  edges,  in  a  few  of  them,  so 
perfect,  that  if  they  were  discovered  associated  with  human  works, 
there  is  no  doubt  that  they  would  have  been  referred  to  the  so- 
called  "stone  period." ' — Extracted  from  the  Minutes  of  the  Welling- 
ton  Philosophical  Society,  February  9,  1869. 


194  FRAGMENTS  OF  SCIENCE. 

sand  was  connected  with  a  reservoir  of  compressed  air, 
the  pressure  being  variable  at  pleasure.  The  hopper 
ended  in  a  long  slit,  from  which  the  sand  was  blown. 
A  plate  of  glass  was  placed  beneath  this  slit,  and  caused 
to  pass  slowly  under  it ;  it  came  out  perfectly  depolished, 
with  a  bright  opalescent  glimmer,  such  as  could  only  be 
produced  by  the  most  careful  grinding.  Every  little 
particle  of  sand  urged  against  the  glass,  having  all  its 
energy  concentrated  on  the  point  of  impact,  formed 
there  a  little  pit,  the  depolished  surface  consisting  of 
innumerable  hollows  of  this  description. 

But  this  was  not  all.  By  protecting  certain  portions 
of  the  surface,  and  exposing  others,  figures  and  tracery 
of  any  required  form  could  be  etched  upon  the  glass. 
The  figiu*es  of  open  iron-work  could  be  thus  copied; 
while  wire-gauze  placed  over  the  glass  produced  a  reti- 
culated pattern.  But  it  required  no  such  resisting 
substance  as  iron  to  shelter  the  glass.  The  patterns  of 
the  finest  lace  could  be  thus  reproduced ;  the  delicate 
filaments  of  the  lace  itself  offering  a  sufficient  protection. 
All  these  effects  have  been  obtained  with  a  simple 
model  of  the  sand-blast  devised  by  my  assistant.  A 
fraction  of  a  minute  suffices  to  etch  upon  glass  a  rich 
and  beautiful  lace  pattern.  Any  yielding  substance 
may  be  employed  to  protect  the  glass.  By  diffusing 
the  shock  of  the  particle,  such  substances  practically 
destroy  the  local  erosive  power.  The  hand  can  bear, 
without  inconvenience,  a  sand-shower  which  would 
pulverise  glass.  Etchings  executed  on  glass  with  suit- 
able kinds  of  ink  are  accurately  worked  out  by  the  sand- 
blast. In  fact,  within  certain  limits,  the  harder  the 
surface,  the  greater  is  the  concentration  of  the  shock, 
and  the  more  effectual  is  the  erosion.  It  is  not  neces- 
sary that  the  sand  should  be  the  harder  substance  of 
the  two ;  corundum,  for  example,  is  much  harder  than 


NIAGAEA.  195 

quartz;  still,  quartz-sand  can  not  only  depolish,  but 
actually  blow  a  bole  tbrougb  a  plate  of  corundum. 
Nay, glass  maybe  depolished  by  tbe  impact  of  fine  shot; 
the  grains  in  this  case  bruising  the  glass,  before  they 
have  time  to  flatten  and  turn  their  energy  into  heat. 

And  here,  in  passing,  we  may  tie  together  one  or 
two  apparently  unrelated  facts.  Supposing  you  turn 
on,  at  the  lower  part  of  a  house,  a  cock  which  is  fed  by 
a  pipe  from  a  cistern  at  the  top  of  the  house,  the  column 
of  water,  from  the  cistern  downwards,  is  set  in  motion. 
By  turning  off  the  cock,  this  motion  is  stopped ;  and 
when  the  turning  off  is  very  sudden,  the  pipe,  if  not 
strong,  may  be  burst  by  the  internal  impact  of  the 
water.  By  distributing  the  turning  of  the  cock  over  half  a 
second  of  time, the  shock  and  danger  of  rupture  maybe 
entirely  avoided.  We  have  here  an  example  of  the  con- 
centration of  energy  in  time.  The  sand-blast  illustrates 
the  concentration  of  energy  in  space.  The  action  of 
flint  and  steel  is  an  illustration  of  the  same  principle. 
The  heat  required  to  generate  the  spark  is  intense ; 
and  the  mechanical  action,  being  moderate,  must,  to 
produce  fire,  be  in  the  highest  degree  concentrated. 
This  concentration  is  secured  by  the  collision  of  hard 
substances.  Calc-spar  will  not  supply  the  place  of  flint, 
nor  lead  the  place  of  steel,  in  the  production  of  fire  by 
collision.  With  the  softer  substances,  the  total  heat 
produced  may  be  greater  than  with  the  hard  ones,  but, 
to  produce  the  spark,  the  heat  must  be  intensely  loca- 
lised. 

We  can,  however,  go  far  beyond  the  mere  depolishing 
of  glass  ;  indeed  I  have  already  said  that  quartz-sand  can 
wear  a  hole  through  corundum.  This  leads  me  to  ex- 
press my  acknowledgments  to  General  Tilghman,^  who 

*  The  absorbent  power,  if  I  may  use  the  phrase,  exerted  by  the 
Industrial  arts  in  the  United  States,  is  forcibly  illustrated  by  the 


196  FKAGMENTS  OF  SCIENCE. 

is  the  inventor  of  the  sand-blast.  To  his  spontaneoua 
kindness  I  am  indebted  for  some  beautiful  illustrations 
of  his  process.  In  one  thick  plate  of  glass  a  figure  has 
been  worked  out  to  a  depth  of  fths  of  an  inch.  A 
second  plate,  |ths  of  an  inch  thick,  is  entirely  per- 
forated. In  a  circular  plate  of  marble,  nearly  half 
an  inch  thick,  open  work  of  most  intricate  and 
elaborate  description  has  been  executed.  It  would  pro- 
bably take  many  days  to  perform  this  work  by  any 
ordinary  process ;  with  the  sand-blast  it  was  accom- 
plished in  an  hour.  So  much  for  the  strength  of  the 
blast ;  its  delicacy  is  illustrated  by  this  beautiful 
example  of  line  engraving,  etched  on  glass  by  means 
of  the  blast. 

This  power  of  erosion,  so  strikingly  displayed  when 
sand  is  urged  by  air,  renders  us  better  able  to  conceive 
its  action  when  urged  by  water.  The  erosive  power  of 
a  river  is  vastly  augmented  by  the  solid  matter  carried 
along  with  it.  Sand  or  pebbles,  caught  in  a  river 
vortex,  can  wear  away  the  hardest  rock  ;  '  potholes '  and 
deep  cylindrical  shafts  being  thus  produced.  An  extra- 
ordinary instance  of  this  kind  of  erosion  is  to  be  seen 
in  the  Val  Tournanche,  above  the  village  of  this  name. 
The  gorge  at  Handeck  has  been  thus  cut  out.  Such 
waterfalls  were  once  frequent  in  the  valleys  of  Switzer- 
land ;  for  hardly  any  valley  is  without  one  or  more 
transverse  barriers  of  resisting  material,  over  which  the 
river  flowing  through  the  valley  once  fell  as  a  cataract. 
Near  Pontresina,  in  the  Engadin,  there  is  such  a  case ; 

rapid  transfer  of  men  like  Mr.  Tilghman  from  the  life  of  the  soldier 
to  that  of  the  civilian.  General  McClellan,  now  a  civil  engineer^ 
whom  I  had  the  honour  of  frequently  meeting  in  New  York,  is  a 
most  eminent  example  of  the  same  kind.  At  the  end  of  the  war, 
indeed,  a  million  and  a  half  of  men  were  thus  drawn,  in  an  as- 
tonidhingly  short  time,  from  military  to  civil  life. 


NIAGAEA.  197 

a  hard  gneiss  being  there  worn  away  to  form  a  gorge, 
through  which  the  river  from  the  Morteratsch  glacier 
rushes.  The  barrier  of  the  Kirchet  above  Meyringen  is 
also  a  case  in  point.  Behind  it  was  a  lake,  derived 
from  the  glacier  of  the  Aar,  and  over  the  barrier  the 
lake  poured  its  excess  of  water.  Here  the  rock,  being 
limestone,  was  in  part  dissolved;  but  added  to  this  we 
had  the  action  of  the  sand  and  gravel  carried  along  by 
the  water,  which,  on  striking  the  rock,  chipped  it 
away  like  the  particles  of  the  sand-blast.  Tims,  by 
solution  and  mechanical  erosion,  the  great  chasm  of  the 
Finsteraarschlucht  was  formed.  It  is  demonstrable 
that  the  water  which  flows  at  the  bottoms  of  such  deep 
fissures  once  flowed  at  the  level  of  their  present  edges, 
and  tumbled  down  the  lower  faces  of  the  bairiers. 
Almost  every  valley  in  Switzerland  furniislies  examples 
of  this  kind ;  the  untenable  hypothesis  of  earthquakes, 
once  so  readily  resorted  to  in  accounting  for  these 
gorges,  being  now  for  the  most  part  abandoned.  To 
produce  the  Canons  of  Western  America,  no  other  cause 
is  needed  than  the  integration  of  effects  individually 
infinitesimal. 

And  now  we  come  to  Niagara.  Soon"  after  Euro- 
peans had  taken  possession  of  the  country,  the  con- 
viction appears  to  have  arisen  that  the  deep  channel  of 
the  river  Niagara  below  the  falls  had  been  excavated 
by  the  cataract.  In  JVIr.  Bakewell's  '  Introduction  to 
Geology,'  the  prevalence  of  this  belief  has  been  referred 
to.  It  is  expressed  thus  by  Professor  Joseph  Henry  in 
the  '  Transactions  of  the  Albany  Institute : '  ^  'In  view- 
ing the  position  of  the  falls,  and  the  features  of  the 
country  round,  it  is  impossible  not  to  be  impressed  with 
the  idea  thajb  this  great  natural  raceway  has  been  formed 

•  Quoted  by  Bakewell, 


198  FRAGMENTS  OF  SCIENCE. 

by  the  continued  action  of  the  irresistible  Niagara,  and 
that  the  falls,  beginning  at  Lewiston,  have,  in  the  course 
of  ages,  worn  back  the  rocky  strata  to  their  present 
site.'  The  same  view  is  advocated  by  Sir  Charles 
Lyell,  by  Mr.  Hall,  by  M.  Agassiz,  by  Professor  Earn- 
say,  indeed  by  most  of  those  who  have  inspected  the 
place. 

A  connected  image  of  the  origin  and  progress  of 
the  cataract  is  easily  obtained.  Walking  northward 
from  the  village  of  Niagara  Falls  by  the  side  of  the 
river,  we  have  to  our  left  the  deep  and  comparatively 
narrow  gorge,  through  which  the  Niagara  flows.  The 
bounding  cliffs  of  this  gorge  are  from  300  to  350  feet 
high.  We  reach  the  whirlpool,  trend  to  the  north-east, 
and  after  a  little  time  gradually  resume  our  northward 
course.  Finally,  at  about  seven  miles  from  the  present 
falls,  we  come  to  the  edge  of  a  declivity,  which  informs 
us  that  we  have  been  hitherto  walking  on  table-land. 
At  some  hundreds  of  feet  below  us  is  a  comparatively 
level  plain,  which  stretches  to  Lake  Ontario.  The  de- 
clivity marks  the  end  of  the  precipitous  gorge  of  the 
Niagara.  Here  the  river  escapes  from  its  steep  mural 
boundaries,  and  in  a  widened  bed  pursues  its  way  to 
the  lake  which  finally  receives  its  waters. 

The  fact  that  in  historic  times,  even  within  the 
memory  of  man,  the  fall  has  sensibly  receded,  prompts 
the  question.  How  far  has  this  recession  gone  ?  At 
what  point  did  the  ledge  which  thus  continually  creeps 
backwards  begin  its  retrograde  course  ?  To  minds 
disciplined  in  such  researches  the  answer  has  been,  and 
will  be-— At  the  precipitous  declivity  which  crossed  the 
Niagara  from  Lewiston  on  the  American  to  Queenston 
on  the  Canadian  side.  Over  this  transverse  barrier  the 
united  affluents  of  all  the  upper  lakes  once  poured  their 


NIAGAEA.  199 

waters,  and  here  the  work  of  erosion  began.  The  dam, 
moreover,  was  demonstrably  of  sufficient  height  to  cause 
the  river  above  it  to  submerge  Goat  Island  ;  and  this 
would  perfectly  account  for  the  finding  by  Sir  Charles 
Lyell,  Mr.  Hall,  and  others,  in  the  sand  and  gravel  of 
the  island,  the  same  fluviatile  shells  as  are  now  found  in 
the  Niagara  Eiver  higher  up.  It  would  also  account  for 
those  deposits  along  the  sides  of  the  river,  the  discovery 
of  which  enabled  Lyell,  Hall,  and  Eamsay  to  reduce  to 
demon stiati on  the  popular  belief  that  the  Niagara 
once  flowed  through  a  shallow  valley. 

The  physics  of  the  problem  of  excavation,  which  I 
made  clear  to  my  mind  before  quitting  Niagara,  are  re- 
vealed by  a  close  inspection  of  the  present  Horseshoe  P^all. 
We  see  evidently  that  the  greatest  weight  of  water  bends 
over  the  very  apex  of  the  Horseshoe.  In  a  passage  in 
his  excellent  chapter  on  Niagara  Falls,  Mr.  Hall  alludes 
to  this  fact.  Here  we  have  the  most  copious  and  the 
most  violent  whirling  of  the  shattered  liquid  ;  here  the 
most  powerful  eddies  recoil  against  the  shale.  From 
this  portion  of  the  fall,  indeed,  the  spray  sometimes 
rises  without  solution  of  continuity  to  the  region  of 
clouds,  becoming  gradually  more  attenuated,  and  passing 
finally  through  the  condition  of  true  cloud  into  invisible 
vapour,  which  is  sometimes  reprecipitated  higher  up. 
All  the  phenomena  point  distinctly  to  the  centre  of 
the  river  as  the  place  of  greatest  mechanical  energy, 
and  from  the  centre  the  vigour  of  the  fall  gradually 
dies  away  towards  the  sides.  The  Horseshoe  form,  with 
the  concavity  facing  downwards,  is  an  obvious  and 
necessary  consequence  of  this  action.  Right  along 
the  middle  of  the  river  the  apex  of  the  curve  pushes 
its  way  backwards,  cutting  along  the  centre  a  deep 
and   comparatively  narrow   groove,  and  draining   the 


14 


200  FRAGMENTS  OF  SCIENCE. 

sides  as  it  passes  them.*  Hence  the  remarkable  dis- 
crepancy between  the  widths  of  the  Niagara  above  and 
below  the  Horseshoe.  All  along  its  course,  from  Lewis- 
ton  Heights  to  its  present  position,  the  form  of  the  fall 
was  probably  that  of  a  horseshoe ;  for  this  is  merely 
the  expression  of  the  greater  depth,  and  consequently 
greater  excavating  power,  of  the  centre  of  the  river. 
The  gorge,  moreover,  varies  in  width,  as  the  depth  of 
the  centre  of  the  ancient  river  varied,  being  narrowest 
where  that  depth  was  greatest. 

The  vast  comparative  erosive  energy  of  the  Horse- 
shoe Fall  comes  strikingly  into  view  when  it  and  the 
American  Fall  are  compared  together.  The  American 
branch  of  the  river  is  cut  at  a  right  angle  by  the 
gorge  of  the  Niagara.  Here  the  Horseshoe  Fall  was 
the  real  excavator.  It  cut  the  rock,  and  formed  the 
precipice,  over  which  the  American  Fall  tumbles.  But 
since  its  formation,  the  erosive  action  of  the  American 
Fall  has  been  almost  nil,  while  the  Horseshoe  has  cut 
its  way  for  500  yards  across  the  end  of  Goat  Island,  and 
is  now  doubling  back  to  excavate  its  channel  parallel  to 
the  length  of  the  island.  This  point,  which  impressed 
me  forcibly,  has  not,  I  have  just  learned,  escaped  the 
acute  observation  of  Professor  Ramsay.*  The  river 
bends;  the  Horseshoe  immediately  accommodates  it- 
self to  the  bending,  and  will  follow  implicitly  the  direc- 
tion of  the  deepest  water  in  the  upper  stream.     Tlie 

*  In  the  discourse  the  excavation  of  the  centre  and  drainage  of 
the  sides  action  was  illustrated  by  a  model  devised  by  my  assistant, 
Mr.  John  Cottrell. 

*  His  words  are :  *  Where  the  body  of  water  is  small  in  the 
American  Fall,  the  edge  has  only  receded  a  few  yards  (where  most 
eroded)  during  the  time  that  the  Canadian  Fall  has  receded  from 
the  north  corner  of  Goat  Island  to  the  innermost  curve  of  the 
Horseshoe  Fall.' — Qv^irterly  Journal  of  Geological  Society ^  May 
1859. 


NIAGAEA.  201 

flexures  of  the  gorge  are  determined  by  those  of  the  river 
channel  above  it.  Were  the  Niagara  centre  above  the  fall 
sinuous,  the  gorge  would  obediently  follow  its  sinuosities. 
Once  suggested,  no  doubt  geographers  will  be  able  to 
point  out  many  examples  of  this  action.  The  Zambesi 
is  thought  to  present  a  great  difficulty  to  the  erosion 
theory,  because  of  the  sinuosity  of  the  chasm  below  the 
Victoria  Falls.  But,  assuming  the  basalt  to  be  of  toler- 
ably uniform  texture,  had  the  river  been  examined 
before  the  formation  of  this  sinuous  channel,  the  present 
zigzag  course  of  the  gorge  below  the  fall  could,  I  am 
persuaded,  have  been  predicted,  while  the  sounding  of 
the  present  river  would  enable  us  to  predict  the  course 
to  be  pursued  by  the  erosion  in  the  future. 

But  not  only  has  the  Niagara  Eiver  cut  the  gorge ; 
it  has  carried  away  the  chips  of  its  own  workshop.  The 
shale,  being  probably  crumbled,  is  easily  carried  away. 
But  at  the  base  of  the  fall  we  find  the  huge  boulders 
already  described,  and  by  some  means  or  other  these  are 
removed  down  the  river.  The  ice  which  fills  the  gorge 
in  winter,  and  which  grapples  with  the  boulders,  has 
been  regarded  as  the  transporting  agent.  Probably  it 
is  so  to  some  extent.  But  erosion  acts  without  ceasing 
on  the  abutting  points  of  the  boulders,  thus  withdrawing 
their  support  and  urging  them  gradually  down  the 
river.  Solution  also  does  its  portion  of  the  work. 
That  solid  matter  is  carried  down  is  proved  by  the  differ- 
ence of  depth  between  the  Niagara  River  and  Lake 
Ontario,  where  the  river  enters  it.  The  depth  falls  from 
72  feet  to  20  feet,  in  consequence  of  the  deposition  of 
solid  matter  caused  by  the  diminished  motion  of  the 
river.* 

*  Near  the  mouth  of  the  gorge  at  Queenston,  the  depth,  ac- 
cording to  the  Admiralty  Chart,  is  180  feet ;  well  within  the  gorge 
it  is  132  feet. 


202 


FRAGMENTS  OF  SCIENCE. 


NIAGARA.  203 

The  annexed  highly  instructive  map  has  been  re- 
duced from  one  published  in  Mr.  Hall's  '  Geology  of 
New  York.'  It  is  based  on  surveys  executed  in  1 842,  by 
Messrs.  Gribson  and  Evershed.  The  ragged  edge  of  the 
American  Fall  north  of  Groat  Island  marks  the  amount 
of  erosion  which  it  has  been  able  to  accomplish,  while 
the  Horseshoe  Fall  was  cutting  its  way  southward  across 
the  end  of  Groat  Island  to  its  present  position.  The 
American  Fall  is  168  feet  high,  a  precipice  cut  down, 
not  by  itself,  but  by  the  Horseshoe  Fall.  The  latter  in 
1842  was  159  feet  high,  and,  as  shown  by  the  map,  is 
already  turning  eastward,  to  excavate  its  gorge  along 
the  centre  of  the  upper  river,  p  is  the  apex  of  the 
Horseshoe,  and  t  marks  the  site  of  the  Terrapin  Tower, 
with  the  promontory  adjacent,  round  which  I  was  con- 
ducted by  Conroy  Probably  since  1842  the  Horse- 
shoe has  worked  back  beyond  the  position  here  assigned 
to  it. 

In  conclusion,  we  may  say  a  word  regarding  the 
proximate  future  of  Niagara.  At  the  rate  of  excavation 
assigned  to  it  by  Sir  Charles  Lyell,  namely,  a  foot  a  year, 
five  thousand  years  or  so  will  carry  the  Horseshoe  Fall 
far  higher  than  Groat  Island.  As  the  gorge  recedes  it 
will  drain,  as  it  has  hitherto  done,  the  banks  right  and 
left  of  it,  thus  leaving  a  nearly  level  terrace  between 
Groat  Island  and  the  edge  of  the  gorge.  Higher  up  it 
will  totally  drain  the  American  branch  of  the  river  ;  the 
channel  of  which  in  due  time  will  become  cultivable 
land.  The  American  Fall  will  then  be  transformed  into 
a  dry  precipice,  forming  a  simple  continuation  of  the 
cliffy  boundary  of  the  Niagara  gorge.  At  the  place 
occupied  by  the  fall  at  this  moment  we  shall  have  the 
gorge  enclosing  a  right  angle,  a  second  whirlpool  being 
the  consequence.  To  those  who  visit  Niagara  a  few 
millenniums  hence  I  leave  the  verification  of  this  pr©- 


204  FRAGMENTS  OF  SCIENCE. 

diction.  All  that  can  be  said  is,  that  if  the  causes 
now  in  action  continue  to  act,  it  will  prove  itself  liter- 
allj  true. 

Postscript. 

A  year  or  bo  after  I  had  quitted  the  United  States, 
a  man  sixty  years  of  age,  while  engaged  in  painting  one 
of  the  bridges  which  connect  Goat  Island  with  the  Three 
Sisters,  slipped  through  the  rails  of  the  bridge  into  the 
rapids,  and  was  carried  impetuously  towards  the  Horse- 
shoe Fall.  He  was  urged  against  a  rock  which  rose 
above  the  water,  and  with  the  grasp  of  desperation  he 
clung  to  it.  The  population  of  the  village  of  Niagara 
Falls  was  soon  upon  the  island,  and  ropes  were  brought, 
but  there  was  none  to  use  them.  In  the  midst  of  the 
excitement,  a  tall  powerful  young  fellow  was  observed 
making  his  way  silently  through  the  crowd.  He  reached 
a  rope ;  selected  from  the  bystanders  a  number  of  men, 
and  placed  one  end  of  the  rope  in  their  hands.  The 
other  end  he  fastened  round  himself,  and  choosing  a 
point  considerably  above  that  to  which  the  man  clung, 
he  plunged  into  the  rapids.  He  was  carried  violently 
downwards,  but  he  caught  the  rock,  secured  the  old 
painter  and  saved  him.  Newspapers  from  all  parts  of 
the  Union  poured  in  upon  me,  describing  this  gallant 
act  of  my  guide  Conroy. 


VIII. 
THE  PARALLEL  ROADS   OF  OLEN  ROT.^ 

The  first  published  allusion  to  the  Parallel  Roads  of 
Glen  Roy  occurs  in  the  appendix  to  the  third  volume  of 
Pennant's  '  Tour  in  Scotland,'  a  work  published  in  1776. 
'  In  the  face  of  these  hills,'  says  this  writer,  '  both  sides 
of  the  glen,  there  are  three  roads  at  small  distances 
from  each  other  and  directly  opposite  on  each  side. 
These  roads  have  been  measured  in  the  complete  parts 
of  them,  and  found  to  be  26  paces  of  a  man  5  feet  10 
inches  high.  The  two  highest  are  pretty  near  each 
other,  about  6(X  yards,  and  the  lowest  double  that 
distance  from  the  nearest  to  it.  They  are  carried  along 
the  sides  of  the  glen  with  the  utmost  regularity,  nearly 
as  exact  as  drawn  with  a  line  of  rule  and  compass.' 

The  correct  heights  of  the  three  roads  of  Grien  Roy 
are  respectively  1150,  1070,  and  860  feet  above  the 
sea.  Hence  a  vertical  distance  of  80  feet  separates  the 
two  highest,  while  the  lowest  road  is  210  feet  below  the 
middle  one. 

These  '  roads  '  are  usually  shelves  or  terraces  formed 
in  the  yielding  drift  which  here  covers  the  slopes  of  the 
mountains.  They  are  all  sensibly  horizontal  and  there- 
fore parallel.  Pennant  accepted  as  reasonable  the 
explanation  of  them  given  by  the  country  people  in  his 

*  A  discourse  delivered  ftt  the  Boyal  Institution  of  Great  Britain 
on  June  9, 1876. 


206  FRAGMENTS  OF    SCIENCK 

time.  They  thought  that  the  roads  '  were  designed  for 
the  chase,  and  that  the  terraces  were  made  after  the 
spots  were  cleared  in  lines  from  wood,  in  order  to  tempt 
the  animals  into  the  open  paths  after  they  were  rouzed, 
in  order  that  they  might  come  within  reach  of  the 
bowmen  who  might  conceal  themselves  in  the  woods 
above  and  below.' 

In  these  attempts  of  '  the  country  people  '  we  have 
an  illustration  of  that  impulse  to  which  all  scientific 
knowledge  is  due — the  desire  to  know  the  causes  of 
things  ;  and  it  is  a  matter  of  surprise  that  in  the  case  of 
the  parallel  roads,  with  their  weird  appearance  chal- 
lenging enquiry,  this  impulse  did  not  make  itself  more 
rapidly .  and  energetically  felt.  Their  remoteness  may 
perhaps  account  for  the  fact  that  until  the  year  1817 
no  systematic  description  of  them,  and  no  scientific 
attempt  at  an  explanation  of  them,  appeared.  In  that 
year  Dr.  MacCulloch,  who  was  then  President  of  the 
Geological  Society,  presented  to  that  Society  a  memoir, 
in  which  the  roads  were  discussed,  and  pronounced  to 
be  the  margins  of  lakes  once  embosomed  in  Glen  Roy. 
Why  there  should  be  three  roads,  or  why  the  lakes 
should  stand  at  these  particular  levels,  was  left  unex- 
plained. 

To  Dr.  MacCulloch  succeeded  a  man,  possibly  not  so 
learned  as  a  geologist,  but  obviously  fitted  by  nature  to 
grapple  with  her  facts  and  to  put  them  in  their 
proper  setting.  I  refer  to  Sir  Thomas  Dick-Lauder, 
who  presented  to  the  Royal  Society  of  Edinburgh,  on 
the  2nd  of  March,  1818,  his  paper  on  the  Parallel  Roads 
of  Glen  Roy.  In  looking  over  the  literature  of  this 
subject,  which  is  now  copious,  it  is  interesting  to  observe 
the  diiTerentiation  of  minds,  and  to  single  out  those 
who  went  by  a  kind  of  instinct  to  the  core  of  the  ques^ 
tion,  from  those  who  erred   in  it,   or   who   learnedly 


THE  PAEALLEL  ROADS  OF  GLEN  ROY.     207 

occupied  themselves  with  its  analogies,  adjuncts,  and 
details.  There  is  no  man,  in  my  opinion,  connected 
with  the  history  of  the  subject,  who  has  shown,  in 
relation  to  it,  this  spirit  of  penetration,  this  force  of 
scientific  insight,  more  conspicuously  than  Sir  Thomas 
Dick-Lauder.  Two  distinct  mental  processes  are  in- 
volved in  the  treatment  of  such  a  question.  Firstly, 
the  faithful  and  sufficient  observation  of  the  data  ;  and 
secondly,  that  higher  mental  process  in  which  the  con- 
structive imagination  comes  into  play,  connecting  the 
separate  facts  of  observation  with  their  common  cause, 
and  weaving  them  into  an  organic  whole.  In  neither 
of  these  requirements  did  Sir  Thomas  Dick-Lauder  fail. 

Adjacent  to  Glen  Eoy  is  a  valley  called  Glen  Gluoy, 
along  the  sides  of  which  ran  a  single  shelf,  or  terrace, 
formed  obviously  in  the  same  manner  as  the  parallel 
roads  of  Glen  Eoy.  The  two  shelves  on  the  opposing 
sides  of  the  glen  were  at  precisely  the  same  level,  and 
Dick-Lauder  wished  to  see  whether,  and  how,  they 
became  united  at  the  head  of  the  glen.  He  followed 
the  shelves  into  the  recesses  of  the  mountains.  The 
bottom  of  the  valley,  as  it  rose,  came  ever  nearer  to 
them,  until  finally,  at  the  head  of  Glen  Gluoy,  he 
reached  a  col,  or  watershed,  of  precisely  the  same 
elevation  as  the  road  which  swept  round  the  glen. 

The  correct  height  of  this  col  is  1170  feet  above 
the  sea ;  that  is  to  say,  20  feet  above  the  highest  road 
in  Glen  Eoy. 

Ftotb.  this  col  a  lateral  branch-valley — Glen  Turrit 
— led  down  to  Glen  Eoy.  Our  explorer  descended  from 
the  col  to  the  highest  road  of  the  latter  glen,  and 
pursued  it  exactly  as  he  had  pursued  the  road  in  Glen 
Gluoy.  For  a  time  it  belted  the  mountain  sides  at  a 
considerable  height  above  the  bottom  of  the  valley ;  but 
this  rose  as  he  proceeded,  coming  ever  nearer  to  the 


208 


FKAGMENTS   OF  SCIENCE. 


highest  shelf,  until  finally  he  reached  a  col,  or  water- 
shed, looking  into  Grlen  Spey,  and  of  precisely  the  same 
elevation  as  the  highest  road  of  Glen  Eoy. 


PAKALLEL  ROADS  OF  GLEN   ROY. 
After  a  Sketch  by  Sib  Thomas  Dick-Laxjder. 

He  then  dropped  down  to  the  lowest  of  these  roads, 
and  followed  it  towards  the  mouth  of  the  glen.  Its 
elevation  above  the  bottom  of  the  valley  gradually 
increased  ;  not  because  the  shelf  rose,  but  because  it 
remained  level  while  the  valley  sloped  downwards.  He 
found  this  lowest  road  doubling  round  the  hills  at  the 
mouth  of  Glen  Roy,  and  running  along  the  sides  of  the 
mountains  which  flank  Glen  Spean.  He  followed  it 
eastwards.  The  bottom  of  the  Spean  Valley,  like  the 
others,  gradually  rose,  and  therefore  gradually  ap- 
proached the  road  on  the  adjacent  mountain-side.  He 
came  to  Loch  Laggan,  the  surface  of  which  rose  almost 


THE  PAKALLEL  ROADS  OF  GLEN  ROY.  209 

to  the  level  of  the  road,  and  beyond  the  head  of  this 
lake  ne  found,  as  in  the  other  two  cases,  a  col,  or  water- 
shed, at  Makul,  of  exactly  the  same  level  as  the  single 
road  in  Glen  Spean,  which,  it  will  be  remembered,  is  a 
continuation  of  the  lowest  road  in  Grlen  Roy. 

Here  we  have  a  series  of  facts  of  obvious  significance 
as  regards  the  solution  of  this  problem.  The  effort  of 
the  mind  to  form  a  coherent  image  from  such  facts 
may  be  compared  with  the  effort  of  the  eyes  to  cause 
the  pictures  of  a  stereoscope  to  coalesce.  For  a  time 
we  exercise  a  certain  strain,  the  object  remaining  vague 
and  indistinct.  Suddenly  its  various  parts  seem  to  run 
together,  the  object  starting  forth  in  clear  and  definite 
relief.  Such,  I  take  it,  was  the  effect  of  his  ponderings 
upon  the  mind  of  Sir  Thomas  Dick-Lauder.  His 
solution  was  this  :  Taking  all  their  features  into  account, 
he  was  convinced  that  water  only  could  have  produced 
the  terraces.  But  how  had  the  water  been  collected  ? 
He  saw  clearly  that,  supposing  the  mouth  of  Glen  Gluoy 
to  be  stopped  by  a  barrier  sufficiently  high,  if  the 
waters  from  the  mountains  flanking  the  glen  were 
allowed  to  collect,  they  would  form  behind  the  barrier 
a  lake,  the  surface  of  which  would  gradually  rise  untU 
it  reached  the  level  of  the  col  at  the  head  of  the  glen. 
The  rising  would  then  cease  ;  the  superfluous  water  of 
Glen  Gluoy  discharging  itself  over  the  col  into  Glen 
Roy.  As  long  as  the  barrier  stopping  the  mouth  of 
Glen  Gluoy  continued  high  enough,  we  should  have  in 
that  glen  a  lake  at  the  precise  level  of  its  shelf,  which 
lake,  acting  upon  the  loose  drift  of  the  flanking  moun- 
tains, would  form  the  shelf  revealed  by  observation. 

So  much  for  Glen  Gluoy.  But  suppose  the  mouth 
of  Glen  Roy  also  stopped  by  a  similar  barrier.  Behind 
it  also  the  water  from  the  adjacent  mountains  would 
collect.      The  surface  of  the  lake  thus  formed  would 


210  FEAGMENTS  OF  SCIENOB. 

gradually  rise,  until  it  had  reached  the  level  of  the 
col  which  divides  Grlen  Eoy  fiom  Grlen  Spey.  Here 
the  rising  of  the  lake  would  cease ;  its  superabundant 
water  being  poured  over  the  col  into  the  valley  of  the 
Spey.  This  state  of  things  would  continue  as  long  as 
a  sufficiently  high  barrier  remained  at  the  mouth  of 
Glen  Roy.  The  lake  thus  dammed  in,  with  its  surface 
at  the  level  of  the  highest  parallel  road,  would  act,  as  in 
Grlen  Gluoy,  upon  the  friable  drift  overspreading  the 
mountains,  and  would  form  the  highest  road  or  terrace 
of  Glen  Roy. 

And  now  let  us  suppose  the  barrier  to  be  so  far 
removed  from  the  mouth  of  Glen  Roy  as  to  establish  a 
connection  between  it  and  the  upper  part  of  Glen  Spean, 
while  the  lower  part  of  the  latter  glen  still  continued 
to  be  blocked  up.  Upper  Glen  Spean  and  Glen  Roy 
would  then  be  occupied  by  a  continuous  lake,  the  level 
of  which  would  obviously  be  determined  by  the  col  at 
the  head  of  Loch  Laggan.  The  water  in  Glen  Roy 
would  sink  from  the  level  it  had  previously  maintained, 
to  the  level  of  its  new  place  of  escape.  This  new  lake- 
surface  would  correspond  exactly  with  the  lowest  parallel 
road,  and  it  would  form  that  road  by  its  action  upon  the 
drift  of  the  adjacent  mountains. 

In  presence  of  the  observed  facts,  this  solution  com- 
mends itself  strongly  to  the  scientific  mind.  The 
question  next  occurs.  What  was  the  character  of  the 
assumed  barrier  which  stopped  the  glens  ?  There  are 
at  the  present  moment  vast  masses  of  detritus  in  certain 
portions  of  Glen  Spean,  and  of  such  detritus  Sir 
Thomas  Dick-Lauder  imagined  his  barriers  to  have 
been  formed.  By  some  unknown  convulsion,  this 
detritus  had  been  heaped  up.  But,  once  given,  and 
once  granted  that  it  was  subsequently  removed  in  the 
manner  indicated,  the  single  road  of  Glen  Gluoy  and 


THE  PARALLEL  KOADS  OF  GLEN  ROY.  211 

the  highest  and  lowest  roads  of  Glen  Eoy  would  be  ex- 
plained in  a  satisfactory  manner. 

To  account  for  the  second  or  middle  road  of  Glen 
Roy,  Sir  Thomas  Dick-Laudef  invoked  a  new  agency. 
He  supposed  that  at  a  certain  point  in  the  breaking 
down  or  waste  of  his  dam,  a  halt  occurred,  the  barrier^ 
holding  its  ground  at  a  particular  level  sufficiently  long 
to  dam  a  lake  rising  to  the  height  of,  and  forming  the 
second  road.  This  point  of  weakness  was  at  once  de- 
tected by  Mr.  Darwin,  and  adduced  by  him  as  proving 
that  the  levels  of  the  cols  did  not  constitute  an  essential 
feature  in  the  phenomena  of  the  parallel  roads.  Though 
not  destroyed.  Sir  Thomas  Dick-Lauder's  theory  was 
seriously  shaken  by  this  argument,  and  it  became  a 
point  of  capital  importance,  if  the  facts  permitted,  to 
remove  such  source  of  weakness.  This  was  done  in 
1847  by  Mr.  David  Milne,  now  Mr.  Milne-Home.  On 
walking  up  Glen  Roy  from  Roy  Bridge,  we  pass  the 
mouth  of  a  lateral  glen,  called  Glen  Glaster,  running 
eastward  from  Glen  Roy.  There  is  nothing  in  this 
lateral  glen  to  attract  attention,  or  to  suggest  that  it 
could  have  any  conspicuous  influence  in  the  production 
of  the  parallel  roads.  Hence,  probably,  the  failure  of 
Sir  Thomas  Dick-Lauder  to  notice  it.  But  Mr.  Milne- 
Home  entered  this  glen,  on  the  northern  side  of  which 
the  middle  and  lowest  roads  are  fairly  shown.  The 
principal  stream  running  through  the  glen  turns  at  a 
certain  point  northwards  and  loses  itself  among  hills 
too  high  to  offer  any  outlet.  But  another  branch  of 
the  glen  turns  to  the  south-east ;  and,  following  up 
this  branch,  Mr.  Milne- Home  reached  a  col,  or  water- 
shed, of  the  precise  level  of  the  second  Glen  Roy  road. 
When  the  barrier  blocking  the  glens  had  been  so  far 
removed  as  to  open  this  col,  the  water  in  Glen  Roy 
would  sink  to  the  level  of  the  second  road.     A  new 


212  FKAGMENTS  OF  SCIENCE. 

lake  of  diminished  depth  would  be  thus  formed,  the 
surplus  water  of  which  would  escape  over  the  Glen 
Grlaster  col  into  Glen  Spean.  The  margin  of  this  new 
lake,  acting  upon  the  dfetrital  matter,  would  form  the 
second  road.  The  theory  of  Sir  Thomas  Dick-Lauder, 
as  regards  the  part  played  by  the  cols,  was  re-riveted 
by  this  new  and  unexpected  discovery. 

I  have  referred  to  Mr.  Darwin,  whose  powerful 
mind  swayed  for  a  time  the  convictions  of  the  scientific 
world  in  relation  to  this  question.  His  notion  was — 
and  it  is  a  notion  which  very  naturally  presents  itself — 
that  the  parallel  roads  were  formed  by  the  sea ;  that 
this  whole  region  was  once  submerged  and  subsequently 
upheaved  ;  that  there  were  pauses  in  the  process  of  up- 
heaval, during  which  these  glens  constituted  so  many 
fiords,  on  the  sides  of  which  the  parallel  terraces  were 
formed.  This  theory  will  not  bear  close  criticism  ;  nor 
is  it  now  maintained  by  Mr.  Darwin  himself.  It  would 
not  account  for  the  sea  being  20  feet  higher  in  Glen 
Gluoy  than  in  Glen  Eoy.  It  would  not  account  for  the 
absence  of  the  second  and  third  Glen  Eoy  roads  from 
Glen  Gluoy,  where  the  moulitain  flanks  are  quite  as  im- 
pressionable as  in  Glen  Eoy.  It  would  not  account  for 
the  absence  of  the  shelves  from  the  other  mountains  in 
the  neighbourhood,  all  of  which  would  have  been 
clasped  by  the  sea  had  the  sea  been  there.  Here  then, 
and  no  doubt  elsewhere,  Mr.  Darwin  has  shown  himself 
to  be  fallible ;  but  here,  as  elsewhere,  he  has  shown 
himself  equal  to  that  discipline  of  surrender  to  evidence 
which  girds  his  intellect  with  such  unassailable  moral 
strength. 

But,  granting  the  significance  of  Sir  Thomas  Dick- 
Lauder's  facts,  and  the  reasonableness,  on  the  whole,  of 
the  views  which  he  has  founded  on  them,  they  will  not 
bear    examination    in   detail.      No   such   barriers    of 


THE  PARALLEL  ROADS  OF  GLEN  ROY.  213 

detritus  as  he  assumed  could  have  existed  without 
leaving  traces  behind  them ;  but  there  is  no  trace  left. 
There  is  detritus  enough  in  Grlen  Spean,  but  not  where 
it  is  wanted.  The  two  highest  parallel  roads  stop 
abruptly  at  dififerent  points  near  the  mouth  of  Glen 
Roy,  but  no  remnant  of  the  barrier  against  which  they 
abutted  is  to  be  seen.  It  might  be  urged  that  the  sub- 
sequent invasion  of  the  valley  by  glaciers  has  swept 
the  detritus  away ;  but  there  have  been  no  glaciers  in 
these  valleys  since  the  disappearance  of  the  lakes. 
Professor  Greikie  has  favoured  me  with  a  drawing  of 
the  Grlen  Spean  '  road '  near  the  entrance  to  Glen 
Trieg.  The  road  forms  a  shelf  round  a  great  mound 
of  detritus  which,  had  a  glacier  followed  the  formation 
of  the  shelf,  must  have  been  cleared  away.  Taking  all 
the  circumstances  into  account,  you  may,  I  think,  with 
safety  dismiss  the  detrital  barrier  as  incompetent  to 
account  for  the  present  condition  of  Glen  Gluoy  and 
Glen  Eoy. 

Hypotheses  in  science,  though  apparently  trans- 
cending experience,  are  in  reality  experience  modified 
by  scientific  thought  and  pushed  into  an  ultra  experien- 
tial region.  At  the  time  that  he  wrote.  Sir  Thomas 
Dick-Lauder  could  not  possibly  have  discerned  the 
cause  subsequently  assigned  for  the  blockage  of  these 
glens.  A  knowledge  of  the  action  of  ancient  glaciers 
was  the  necessary  antecedent  to  the  new  explanation, 
and  experience  of  this  nature  was  not  possessed  by  the 
distinguished  writer  just  mentioned.  The  extension  of 
Swiss  glaciers  far  beyond  their  present  limits,  was  first 
made  known  by  a  Swiss  engineer  named  Venetz,  who 
established,  by  the  marks  they  had  left  behind  them, 
their  former  existence  in  places  which  they  had  long 
forsaken.  The  subject  of  glacier  extension  was  subset 
quentlj  followed    up  with  distinguished  success    by 


214  FKAaMENTS  OF  SCIENCE. 

Charpentier,  Studer,  and  others.  With  characteristic 
vigour  Agassiz  grappled  with  it,  extending  his  obser- 
vations far  beyond  the  domain  of  Switzerland.  He 
came  to  this  country  in  1840,  and  found  in  various 
places  indubitable  marks  of  ancient  glacier  action. 
England,  Scotland,  Wales,  and  Ireland  he  proved  to 
have  once  given  birth  to  glaciers.  He  visited  Glen 
Eoy,  surveyed  the  sm*rounding  neighbourhood,  and 
pronounced,  as  a  consequence  of  his  investigation,  the 
barriers  which  stopped  the  glens  and  produced  the 
parallel  roads  to  have  been  barriers  of  ice.  To  Mr. 
Jamieson,  above  all  others,  we  are  indebted  for  the 
thorough  testing  and  confirmation  of  this  theory. 

And  let  me  here  say  that  Agassiz  is  only  too  likely 
to  be  misrated  and  misjudged  by  those  who,  though 
accurate  within  a  limited  sphere,  fail  to  grasp  in  their 
totality  the  motive  powers  invoked  in  scientific  inves- 
tigation. True  he  lacked  mechanical  precision,  but  he 
abounded  in  that  force  and  freshness  of  the  scientific 
imagination  which  in  some  sciences,  and  probably  in 
some  stages  of  all  sciences,  are  essential  to  the  creator 
of  knowledge.  To  Agassiz  was  given,  not  the  art  of 
the  refiner,  but  the  instinct  of  the  discoverer,  and  the 
strength  of  the  delver  who  brings  ore  from  the  recesses 
of  the  mine.  That  ore  may  contain  its  share  of  dross, 
but  it  also  contains  the  precious  metal  which  gives 
employment  to  the  refiner,  and  without  which  his 
occupation  would  depart. 

Let  us  dwell  for  a  moment  upon  this  subject  of 
ancient  glaciers.  Under  a  flask  containing  water,  in 
which  a  thermometer  is  immersed,  is  placed  a  Bunsen's 
lamp.  The  water  is  heated,  reaches  a  temperature  of 
212°,  and  then  begins  to  boil.  The  rise  of  the  ther- 
mometer then  ceases,  although  heat  continues  to  be 
poured  by  the  lamp  into  the  water.     What  becomes  of 


THE  PAEALLEL  ROADS  OF  GLEN  EOY.  215 

that  heat  ?  We  know  that  it  is  consumed  in  the  mo- 
lecular work  of  vaporization.  In  the  experiment  here 
arranged,  the  steam  passes  from  the  flask  through  a 
tube  into  a  second  vessel  kept  at  a  low  temperature. 
Here  it  is  condensed,  and  indeed  congealed  to  ice,  the 
second  vessel  being  plunged  in  a  mixture  cold  enough 
to  freeze  the  water.  As  a  result  of  the  process  we 
obtain  a  mass  of  ice.  That  ice  has  an  origin  very 
antithetical  to  its  own  character.  Though  cold,  it  is 
the  child  of  heat.  If  we  removed  the  lamp,  there 
would  be  no  steam,  and  if  there  were  no  steam  there 
would  be  no  ice.  The  mere  cold  of  the  mixture  sur- 
rounding the  second  vessel  would  not  produce  ice.  The 
cold  must  have  the  proper  material  to  work  upon ;  and 
this  material — aqueous  vapour — is,  as  we  here  see,  the 
direct  product  of  heat. 

It  is  now,  I  suppose,  fifteen  or  sixteen  years  since  I 
found  myself  conversing  with  an  illustrious  philosopher 
regarding  that  glacial  epoch  which  the  researches  of 
Agassiz  and  others  had  revealed.  This  profoundly 
thoughtful  man  maintained  the  fixed  opinion  that,  at  a 
certain  stage  in  the  history  of  the  solar  system,  the 
sun's  radiation  had  suffered  diminution,  the  glacial 
epoch  being  a  consequence  of  this  solar  chill.  The 
celebrated  French  mathematician  Poisson  had  another 
theory.  Astronomers  have  shown  that  the  solar  system 
moves  through  space,  and  '  the  temperature  of  space '  is 
a  familiar  expression  with  scientific  men.  It  was  con- 
sidered probable  by  Poisson  that  our  system,  during  its 
motion,  had  traversed  portions  of  space  of  different 
temperatures ;  and  that,  during  its  passage  through 
one  of  the  colder  regions  of  the  universe,  the  glacial 
epoch  occurred.  Notions  such  as  these  were  more  or 
less  current  everywhere  not  many  years  ago,  and  I 
therefore  thought  it  worth  while  to  show  how  incom- 

15 


216  FRAGMENTS  OF  SCIENCE. 

plete  they  were.  Suppose  the  temperature  of  oui 
planet  to  be  reduced,  by  the  subsidence  of  solar  heat, 
the  cold  of  space,  or  any  other  cause,  say  one  hundred 
degrees.  Four-and-twenty  hours  of  such  a  chill  would 
bring  down  as  snow  nearly  all  the  moisture  of  our 
atmosphere.  But  this  would  not  produce  a  glacial 
epoch.  Such  an  epoch  would  require  the  long-continued 
generation  of  the  material  from  which  the  ice  of  glaciers 
is  derived.  Mountain  snow,  the  nutriment  of  glaciers, 
is  derived  from  aqueous  vapour  raised  mainly  from  the 
tropical  ocean  by  the  sun.  The  solar  fire  is  as  neces- 
sary a  factor  in  the  process  as  our  lamp  in  the  experi- 
ment referred  to  a  moment  ago.  Nothing  is  easier 
than  to  calculate  the  exact  amount  of  heat  expended 
by  the  sun  in  the  production  of  a  glacier.  It  would, 
as  I  have  elsewhere  shown,'  raise  a  quantity  of  cast 
iron  five  times  the  weight  of  the  glacier  not  only  to  a 
white  heat,  but  to  its  point  of  fusion.  If,  as  I  have 
already  urged,  instead  of  being  filled  with  ice,  the 
valleys  of  the  Alps  were  filled  with  white-hot  metal,  of 
quintuple  the  mass  of  the  present  glaciers,  it  is  the 
heat,  and  not  the  cold,  that  would  arrest  oiu*  attention 
and  solicit  our  explanation.  The  process  of  glacier 
making  is  obviously  one  of  distillation,  in  which  the 
fire  of  the  sun,  which  generates  the  vapour,  plays  as 
essential  a  part  as  the  cold  of  the  mountains  which 
condenses  it.^ 

It  was  their  ascription  to  glacier  action  that  first 

*  *  Heat  a  Mode  of  Motion,'  fifth  edition,  chap.  vi. :  Forms  of 
Water,  §§55  and  56. 

*  In  Lyell's  excellent  *  Principles  of  Geology,'  the  remark  occurs 
that '  several  writers  have  fallen  into  the  strange  error  of  supposing 
that  the  glacial  period  must  have  been  one  of  higher  mean  tempe- 
rature than  usual.'  The  really  strange  error  was  the  forgetfulness 
of  the  fact  that  without  the  heat  the  substance  necessary  to  the 
production  of  glaciers  would  be  wanting. 


THE  PARALLEL  ROADS  OF  GLEN  ROY.    217 

gave  the  parallel  roads  of  Grlen  Eoy  an  interest  in  my 
eyes;  and  in  1867,  with  a  view  to  self-instruction,  I 
made  a  solitary  pilgrimage  to  the  place,  and  explored 
pretty  thoroughly  the  roads  of  the  principal  glen.  I 
traced  the  highest  road  to  the  col  dividing  Grlen  Eoy 
from  Grlen  Spey,  and,  thanks  to  the  civility  of  an 
Ordnance  surveyor,  I  was  enabled  to  inspect  some  of 
the  roads  with  a  theodolite,  and  to  satisfy  myself  re- 
garding the  common  level  of  the  shelves  at  opposite 
sides  of  the  valley.  As  stated  by  Pennant,  the  width 
of  the  roads  amounts  sometimes  to  more  than  twenty 
yards ;  but  near  the  head  of  Grlen  Eoy  the  highest  road 
ceases  to  have  any  width,  for  it  runs  along  the  face  of 
a  rock,  the  effect  of  the  lapping  of  the  water  on  the 
more  friable  portions  of  the  rock  being  perfectly 
distinct  to  this  hour.  My  knowledge  of  the  region 
was,  however,  far  from  complete,  and  nine  years  had 
dimmed  the  memory  even  of  the  portion  which  had  been 
thoroughly  examined.  Hence  my  desire  to  see  the 
roads  once  more  before  venturing  to  talk  to  you  about 
them.  The  Easter  holidays  of  1876  were  to  be  devoted 
to  this  purpose;  but  at  the  last  moment  a  telegram 
from  Eoy  Bridge  informed  me  that  the  roads  were 
snowed  up.  Finding  books  and  memories  poor  substi- 
tutes for  the  flavour  of  facts,  I  resolved  subsequently 
to  make  another  effort  to  see  the  roads.  Accordingly 
last  Thursday  fortnight,  after  lecturing  here,  I  packed 
up,  and  started  (not  this  time  alone)  for  the  North. 
Next  day  at  noon  my  wife  and  I  found  ourselves  at 
Dalwhinnie,  whence  a  drive  of  some  five-and-  thirty  miles 
brought  us  to  the  excellent  hostelry  of  Mr.  Macintosh, 
at  the  mouth  of  Grlen  Eoy. 

We  might  have  found  the  hills  covered  with  mist, 
which  would  have  wholly  defeated  us ;  but  Nature  was 
good-natured,  and  we  had  two  successful  working  days 


218  FRAGMENTS  OF  SCIENCE. 

among  the  hills.  Guided  by  the  excellent  ordnance  map 
of  the  region,  on  the  Saturday  morning  we  went  up  the 
glen,  and  on  reaching  the  stream  called  Allt  Bhreac 
Achaidh  faced  the  hills  to  the  west.  At  the  watershed 
between  Glen  Roy  and  Glen  Fintaig  we  bore  north  wards, 
struck  the  ridge  above  Glen  Gluoy,  came  in  view  of  its 
road,  which  we  persistently  followed  as  long  as  it  con- 
tinued visible.  It  is  a  feature  of  all  the  roads  that  they 
vanish  before  reaching  the  cols  over  which  fell  the  waters 
of  the  lakes  which  formed  them.  One  reason  doubtless 
is  that  at  their  upper  ends  the  lakes  were  shallow,  and 
incompetent  on  this  accoimt  to  raise  wavelets  of  any 
strength  to  act  upon  the  mountain  drift.  A  second 
reason  is  that  they  were  land-locked  in  the  higher 
portions  and  protected  from  the  south-westerly  winds, 
the  stillness  of  their  waters  causing  them  to  produce  but 
a  feeble  impression  upon  the  mountain  sides.  From 
Glen  Gluoy  we  passed  down  Glen  Turrit  to  Glen 
Eoy,  and  through  it  homewards,  thus  accomplishing 
two  or  three  and  twenty  miles  of  rough  and  honest 
work. 

Next  day  we  thoroughly  explored  Glen  Glaster, 
following  its  two  roads  as  far  as  they  were  visible.  We 
reached  the  col  discovered  by  Mr.  Milne-Home,  which 
stands  at  the  level  of  the  middle  road  of  Glen  Roy. 
Thence  we  crossed  southwards  over  the  mountain  Creag 
Dhubh,  and  examined  the  erratic  blocks  upon  its  sides, 
and  the  ridges  and  mounds  of  moraine  matter  which 
cumber  the  lower  flanks  of  the  mountain.  The  obser- 
vations of  Mr.  Jamieson  upon  this  region,  including  the 
mouth  of  Glen  Trieg,  are  in  the  highest  degree  interest- 
ing. We  entered  Glen  Spean,  and  continued  a  search 
begun  on  the  evening  of  our  arrival  at  Roy  Bridge — 
the  seaich,  namely,  for  glacier  polishings  and  markings. 
We  did  not  find  them  copious,  but  they  are  indubitable 


THE  PAEALLEL  ROADS  OF  GLEN  ROY.  S19 

One  of  the  proofs  most  convenient  for  reference,  is  a 
great  rounded  rock  by  the  roadside,  1,000  yards  east  of 
the  milestone  marked  three-quarters  of  a  mile  from 
Eoy  Bridge.  F'arther  east  other  cases  occur,  and  they 
leave  no  doubt  upon  the  mind  that  Glen  Spean  was  at 
one  time  filled  by  a  great  glacier.  To  the  disciplined 
eye  the  aspect  of  the  mountains  is  perfectly  conclusive 
on  this  point ;  and  in  no  position  can  the  observer  more 
readily  and  thoroughly  convince  himself  of  this  than  at 
tlie  head  of  Glen  Glaster,  The  dominant  hills  here  are 
all  intensely  glaciated. 

But  the  great  collecting  ground  of  the  glaciers 
which  dammed  the  glens  and  produced  the  parallel 
roads,  were  the  mountains  south  and  west  of  Glen 
Spean.  The  monarch  of  these  is  Ben  Nevis,  4,370  feet 
high.  The  position  of  Ben  Nevis  and  his  colleagues,  in 
reference  to  the  vapour-laden  winds  of  the  Atlantic,  is 
a  point  of  the  first  importance.  It  is  exactly  similar  to 
that  of  Carrantual  and  the  Macgillicuddy  Reeks  in  the 
south-west  of  Ireland.  These  mountains  are,  and  were, 
the  first  to  encounter  the  south-western  Atlantic  winds, 
and  the  precipitation,  even  at  present,  in  the  neighbour- 
hood of  Killarney,  is  enormous.  The  winds,  robbed  of 
their  vapour,  and  charged  with  the  heat  set  free  by  its 
precipitation,  pursue  their  direction  obliquely  across 
Ireland ;  and  the  effect  of  the  drying  process  may  be 
understood  by  comparing  the  rainfall  at  Cahirciveen 
with  that  at  Portarlington.  As  found  by  Dr.  Lloyd, 
the  ratio  is  as  59  to  21 — fifty-nine  inches  annually  at 
Cahirciveen  to  twenty-one  at  Portarlington.  During 
the  glacial  epoch  this  vapour  fell  as  snow,  and  the  con- 
sequence was  a  system  of  glaciers  which  have  left  traces 
and  evidences  of  the  most  impressive  character  in  the 
region  of  the  Killarney  Lakes.  I  have  referred  in  other 
places  t^  the  great  glacier  which,  descending  from  the 


220  FRAGMENTS  OE  SCIENCE. 

Reeks,  moved  through  the  Black  Valley,  took  posses- 
sion of  the  lake-basins,  and  left  its  traces  on  every  rock 
and  island  emergent  from  the  waters  of  tlie  upper  lake. 
They  are  all  conspicuously  glaciated.  Not  in  Switzer- 
land itself  do  we  find  clearer  traces  of  ancient  glacier 
action. 

What  the  Macgillicuddy  Reeks  did  in  Ireland,  Ben 
Nevis  and  the  adjacent  mountains  did,  and  continue  to 
do,  in  Scotland.  We  had  an  example  of  this  on  the 
morning  we  quitted  Eoy  Bridge.  From  the  bridge 
westward  rain  fell  copiously,  and  the  roads  were  wet ; 
but  the  precipitation  ceased  near  Loch  Laggan,  whence 
eastward  the  roads  were  dry.  Measured  by  the  gauge, 
the  rainfall  at  P'ort  William  is  86  inches,  while  at 
Laggan  it  is  only  46  inches  annually.  The  difference 
between  west  and  east  is  forcibly  brought  out  by  obser- 
vations  at  the  two  ends  of  the  Caledonian  Canal.  Fort 
William  at  the  south-western  end  has,  as  just  stated, 
86  inches,  while  Culloden,  at  its  north-eastern  end,  has 
only  24.  To  the  reseo.rches  of  that  able  and  accom- 
plished meteorologist,  Mr.  Buchan,  we  are  indebted  for 
these  and  other  data  of  the  most  interesting  and 
valuable  kind. 

Adhering  to  the  facts  now  presented  to  us,  it  is  not 
difficult  to  restore  in  idea  the  process  by  which  the 
glaciers  of  Lochaber  were  produced  and  the  glens 
dammed  by  ice.  When  the  cold  of  the  glacial  epoch 
began  to  invade  the  Scottish  hills,  the  sun  at  the  same 
time  acting  with  sufficient  power  upon  the  tropical 
ocean,  the  vapours  raised  and  drifted  on  to  these 
northern  mountains  were  more  and  more  converted 
into  snow.  This  slid  down  the  slopes,  and  from  every 
valley,  strath,  and  corry,  south  of  Glen  Spean,  glaciers 
were  poured  into  that  glen.  The  two  great  factors 
here  brought  into  play  are  the  nutrition  of  the  glaciers 


THE  PARALLEL  ROADS  OF  GLEN  ROY.  221 

by  the  frozen  material  above,  and  their  consumption  in 
the  milder  air  below.  For  a  period  supply  exceeded 
consumption,  and  the  ice  extended,  filling  Glen  Spean 
to  an  ever-increasing  height,  and  abatting  against  the 
mountains  to  the  north  of  that  glen.  But  why,  it  may 
be  asked,  should  the  valleys  south  of  Glen  Spean  be 
receptacles  of  ice  at  a  time  when  those  north  of  it  were 
receptacles  of  water?  The  answer  is  to  be  found  in 
the  position  and  the  greater  elevation  of  the  mountains 
south  of  Glen  Spean.  They  first  receivetl  the  loads  of 
moisture  carried  by  the  Atlantic  winds,  and  not  until 
they  had  been  in  part  dried,  and  also  warmed  by  the 
liberation  of  their  latent  heat,  did  these  winds  touch 
the  hills  north  of  the  Glen. 

An  instructive  observation  bearing  upon  this  point 
is  here  to  be  noted.  Had  our  visit  been  in  the  winter 
we  should  have  found  all  the  mountains  covered ;  had 
it  been  in  the  summer  we  should  have  found  the  snow 
all  gone.  But  happily  it  was  at  a  season  when  the 
aspect  of  the  mountains  north  and  south  of  Glen  Spean 
exhibited  their  relative  powers  as  snow  collectors. 
Scanning  the  former  hills  from  many  points  of  view* 
we  were  hardly  able  to  detect  a  fleck  of  snow,  while 
heavy  swaths  and  patches  loaded  the  latter.  Were  the 
glacial  epoch  to  return,  the  relation  indicated  by  this 
observation  would  cause  Glen  Spean  to  be  fiUed  with 
glaciers  from  the  south,  while  the  hills  and  valleys  on 
the  noith,  visited  by  warmer  and  drier  winds,  would 
remain  comparatively  free  from  ice.  This  flow  from 
the  south  would  be  reinforced  from  the  west,  and  as 
long  as  the  STipply  was  in  excess  of  the  consumption 
the  glaciers  would  extend,  the  dams  which  closed  the 
glens  increasing  in  height.  By-and-by  supply  and  con- 
sumption becoming  approximately  equal,  the  height  of 
the  glacier  barriers  would  remain  constant.     Then,  if 


222  FRAGMENTS  OF  SCIENCE, 

milder  weathei  set  in,  consumption  would  be  in  excess, 
a  lowering  of  the  barriers  and  a  retreat  of  the  ice  being 
the  consequence.  But  for  a  long  time  the  conflict 
between  supply  and  consumption  would  continue,  re- 
tarding indefinitely  the  disappearance  of  the  barriers, 
and  keeping  the  imprisoned  lakes  in  the  northern 
glens.  But  however  slow  its  retreat,  the  ice  in  the 
long  run  would  be  forced  to  yield.  The  dam  at  the 
mouth  of  Glen  Roy,  which  probably  entered  the  glen 
Bufficiently  far  to  block  up  Glen  Glaster,  would 
gradually  retreat.  Glen  Glaster  and  its  col  being 
opened,  the  subsidence  of  the  lake  eighty  feet,  from 
the  level  of  the  highest  to  that  of  the  second  parallel 
road,  would  follow  as  a  consequence.  I  think  this  the 
most  probable  course  of  things,  but  it  is  also  possible 
that  Glen  Glaster  may  have  been  blocked  by  a  glacier 
from  Glen  Trieg.  The  ice  dam  continuing  to  retreat, 
at  length  permitted  Glen  Roy  to  connect  itself  with 
upper  Glen  Spean.  A  continuous  lake  then  filled  both 
glens,  the  level  of  which,  as  already  explained,  was 
determined  by  the  col  at  Makul,  above  the  head  of 
Loch  Laggan.  The  last  to  yield  was  the  portion  of  the 
glacier  which  derived  nutrition  from  Ben  Nevis,  and 
probably  also  from  the  mountains  north  and  south  of 
Loch  Arkaig,  But  it  at  length  yielded,  and  the  waters 
in  the  glens  resumed  the  courses  which  they  pursue 
to-day. 

For  the  removal  of  the  ice  barriers  no  cataclysm  is 
to  be  invoked ;  the  gradual  melting  of  the  dam  would 
produce  the  entire  series  of  phenomena.  In  sinking 
from  col  to  col  the  water  would  flow  over  a  gradually 
melting  barrier,  the  surface  of  the  imprisoned  lake  not 
remaining  sufficiently  long  at  any  particular  level  to 
produce  a  shelf  comparable  to  the  parallel  roads.  By 
t/emporary  halts  in  the  process  of  melting  due  to  atmo- 


THE  PARALLEL  ROADS  OP  GLEN  ROY.    2S3 

spheric  conditions  or  to  the  character  of  the  dam  itself, 
or  through  local  softness  in  the  drift,  small  pseudo- 
terraces  would  be  formed,  which,  to  the  perplexity  of 
some  observers,  are  seen  upon  the  flanks  of  the  glens 
to-day. 

In  presence  then  of  the  fact  that  the  barriers  which 
stopped  these  glens  to  a  height,  it  may  be,  of  1,500 
feet  above  the  bottom  of  Glen  Spean,  have  dissolved 
and  left  not  a  wreck  behind ;  in  presence  of  the  fact, 
insisted  on  by  Professor  Greikie,  that  barriers  of  detritus 
would  undoubtedly  have  been  able  to  maintain  them- 
selves had  they  ever  been  there ;  in  presence  of  the  fact 
that  great  glaciers  once  most  certainly  filled  these 
valleys — that  the  whole  region,  as  proved  by  Mr. 
Jamieson,  is  filled  with  the  traces  of  their  action ;  the 
theory  which  ascribes  the  parallel  roads  to  lakes 
dammed  by  barriers  of  ice  has,  in  my  opinion,  a 
degree  of  probability  on  its  side  which  amounts  to  a 
practical  demonstration  of  its  truth. 

Into  the  details  of  the  terrace  formation  I  do  not 
enter.  Mr.  Darwin  and  Mr.  Jamieson  on  the  one  side, 
and  Sir  John  Lubbock  on  the  other,  deal  with  true 
causes.  The  terraces,  no  doubt,  are  due  in  part  to  the 
descending  drift  arrested  by  the  water,  and  in  part  to 
the  fretting  of  the  wavelets,  and  the  rearrangement  of 
the  stirred  detritus,  along  the  belts  of  contact  of  lake 
and  hill.  The  descent  of  matter  must  have  been 
frequent  when  the  drift  was  unbound  by  the  rootlets 
which  hold  it  together  now.  In  some  cases,  it  may  be 
remarked,  the  visibility  of  the  roads  is  materially  aug- 
mented by  differences  of  vegetation.  The  grass  upon 
the  terraces  is  not  always  of  the  same  character  as  that 
above  and  below  them,  while  on  heather-covered  hills 
the  absence  of  the  dark  shrub  from  the  roads  greatly 
enhances  their  conspicuousness. 


224  FRAGMENTS  OF  SCIENCE. 

The  annexed  sketch  of  a  model  (p.  225)  will  enable 
the  reader  to  grasp  the  essential  features  of  the  problem 
and  its  solution.  Glen  Gluoy  and  Glen  Roy  are  lateral 
valleys  which  open  into  Glen  Spean.  Let  us  suppose 
Glen  Spean  filled  from  v  to  w  with  ice  of  a  uniform 
elevation  of  1,500  feet  above  the  sea,  the  ice  not 
filling  the  upper  part  of  that  glen.  The  ice  would 
thrust  itself  for  some  distance  up  the  lateral  valleys, 
closing  all  their  mouths.  The  streams  from  the  moun- 
tains right  and  left  of  Glen  Gluoy  would  pour  their 
waters  into  that  glen,  forming  a  lake,  the  level  of  which 
would  be  determined  by  the  height  of  the  col  at  a,  11 70 
feet  above  the  sea.  Over  this  col  the  water  would  flow 
into  Glen  Roy.  But  in  Glen  Roy  it  could  not  rise  higher 
than  1150  feet,  the  height  of  the  col  at  b,  over  which  it 
would  flow  into  Glen  Spey. 

The  water  halting  at  these  levels  for  a  sufficient 
time,  would  form  the  single  road  in  Glen  Gluoy  and  the 
highest  road  in  Glen  Roy.  This  state  of  things  would 
(jontinue  as  long  as  the  ice  dam  was  sufficiently  high  to 
dominate  the  cols  at  a  and  b  ;  but  when  through  change 
of  climate  the  gradually  sinking  dam  reached,  in  succes- 
sion, the  levels  of  these  cols,  the  water  would  then  begin 
to  flow  over  the  dam  instead  of  over  the  cols.  Let  us 
suppose  the  wasting  of  the  ice  to  continue  until  a  con- 
nection was  established  between  Glen  Roy  and  Glen 
Glaster,  a  common  lake  would  then  fill  both  these  glens, 
the  level  of  which  would  be  determined  by  that  of  the 
col  c,  over  which  the  water  would  pour  for  an  indefinite 
period  into  Glen  Spean.  During  this  period  the  second 
Glen  Roy  road  and  the  highest  road  of  Glen  Glaster 
would  be  formed.  The  ice  subsiding  stiU  further,  a 
connection  would  eventually  be  established  between 
Glen  Roy,  Glen  Glaster,  and  the  upper  part  of  Glen 
Spean.     A  common  lake  would  fill  all  three  glens,  the 


THE  PARALLEL  ROADS  OF  GLEN  ROY.    225 


226  FRAGMENTS  OF  SCIENCE. 

level  of  which  would  be  that  of  the  col  d,  over  which  foi 
an  indefinite  period  the  lake  would  pour  its  water. 
During  this  period  the  lowest  Grien  Eoj  road,  which  is 
common  also  to  Glen  Glaster  and  Glen  Spean,  would  be 
formed.  Finally,  on  the  disappearance  of  the  ice  from 
the  lower  part  of  Glen  Spean  the  waters  would  flow 
down  their  respective  valleys  as  they  do  to-day. 

Eeviewing  our  work,  we  find  three  considerable 
steps  to  have  marked  the  solution  of  the  problem  of  the 
Parallel  Roads  of  Glen  Roy.  The  first  of  these  was  taken 
by  Sir  Thomas  Dick-Lauder,  the  second  was  the  pregnant 
conception  of  Agassiz  regarding  glacier  action,  and  the 
third  was  the  testing  and  verification  of  this  concep- 
tion by  the  very  thorough  researches  of  Mr.  Jamieson. 
No  circumstance  or  incident  connected  with  this  dis- 
course gives  me  greater  pleasure  than  the  recognition 
of  the  value  of  these  researches.  They  are  marked 
throughout  by  unflagging  industry,  by  novelty  and 
acutoness  of  observation,  and  by  reasoning  power  of  a 
high  and  varied  kind.  These  pages  had  been  returned 
'  for  press '  when  I  learned  that  tlie  relation  of  Ben 
Nevis  and  his  colleagues  to  the  vapour-laden  winds  of 
the  Atlantic  had  not  escaped  Mr.  Jamieson.  To  him 
obviously  the  exploration  of  Lochaber,  and  the  develop- 
ment of  the  theory  of  the  Parallel  Roads,  has  been  a 
labour  of  love. 

Thus  ends  our  rapid  survey  of  this  brief  episode 
in  the  physical  history  of  the  Scottish  hills, — brief, 
that  is  to  say,  in  comparison  with  the  immeasurable 
lapses  of  time  through  which,  to  produce  its  varied 
structure  and  appearances,  our  planet  must  have 
passed.  In  the  survey  of  such  a  field  two  things  are 
specially  worthy  to  be  taken  into  account — the  widen- 
ing of  the  intellectual  horizon  and  the  reaction  of  ex- 
panding knowledge  upon  the  intellectual  organ  itself 


THE  PAKALLEL  ROADS  OF  GLEN  ROY.  227 

At  first,  as  in  the  case  of  ancient  glaciers,  through 
sheer  want  of  capacity,  the  mind  refuses  to  take  in 
revealed  facts.  But  by  degrees  tlie  steady  contem- 
plation of  these  facts  so  strengthens  and  expands  the 
intellectual  powers,  that  where  truth  once  could  not 
find  an  entrance  it  eventually  finds  a  home.* 

A  map  of  the  district,  with  the  parallel  roads  shown 
in  red,  is  annexed. 


LITERATURE   OF  THE   SUBJECT. 

Thomas  Pennant. — A  Tour  in  Scotland.    Vol.  iii.  1776,  p.  394. 
John  MacCulloch. — On  the  Parallel  Roads  of  Glen  Roy.    Geol. 

Soc.  Trans,  vol.  iv.  1817,  p.  314. 
Thomas  Lauder  Dick  (afterwards  Sib  Thomas  Dick-Laudee, 

Bart.)— On  the  Parallel  Roads  of  Lochaber.     Edin.  Roy.  Soc 

Trans.  1818,  vol.  ix.  p.  1. 
Charles  Darwin. — Observations  on  the  Parallel  Roads  of  Glen 

Roy,  and  of  the  other  parts  of  Lochaber  in  Scotland,  with  an 

attempt  to  prove  that  they  are  of  marine  origin.    Phil.  Trans. 

1839,  vol.  cxxix.  p.  39. 
Sir   Charles  Lyell. — Elements  of  Geology.      Second   edition, 

1841. 
Louis  Agassiz. — The  Glacial  Theory  and  its  Recent  Progress — 

Parallel  Terraces.     Edin.  New  Phil.  Journal,  1842,  vol.  xxxiii. 

p.  236. 

•  The  formation,  connection,  successive  subsidence,  and  final 
disappearance  of  the  glacial  lakes  of  Lochaber  were  illustrated  in 
the  discourse  here  reported  by  the  model  just  described,  constructed 
under  the  supervision  of  my  assistant,  Mr.  John  Cottrell.  Glen 
Gluoy  with  its  lake  and  road  and  the  cataract  over  its  col ;  Glen 
Roy  and  its  three  roads  with  their  respective  cataracts  at  the  head 
of  Glen  Spey,  Glen  Glaster,  and  Glen  Spean,  were  aU  represented. 
The  successive  shiftings  of  the  barriers,  which  were  formed  of  plate 
glass,  brought  each  successive  lake  and  its  corresponding  road  into 
view,  while  the  entire  removal  of  the  barriers  caused  the  streams 
to  flow  down  the  glens  of  the  model  as  they  flow  down  the  real 
glens  of  to-day. 


228  FKAGMENTS  OF  SCIENCE. 

David  Milnb  (afterwards  David  Milne-Home).— On  the  Parallel 

Eoads  of  Lochaber ;  with  Eemarks  on  the  Change  of  Relative 

Levels  of   Sea  and  Land  in  Scotland,  and  on  the  Detrital 

Deposits  in  that  Country.    Edin.  Roy.  Soc.  Trans.  1847,  vol. 

xvi.  p.  395. 
Robert  Chambers. — Ancient  Sea  Margins.    Edinburgh,  1848. 
H.  D.  Rogers.— On  the  Parallel  Roads  of  Glen  Roy.    Royal  List 

Proceedings,  1861,  vol.  iii,  p.  341. 
Thomas  F.  Jamieson. — On  the  Parallel  Roads  of  Glen  Roy,  and 

their  Place  in  the  History  of  the  Glacial  Period.      Quart. 

Journal  Geol.  Soc.  1863,  vol.  xix.  p.  235. 
Sir  Charles  Lyell. — Antiquity  of  Man.    1863,  p.  263. 
Rev.  R.  B.  Watson. — On  the  Marine  Origin  of  the  Parallel  Roads 

of  Glen  Roy.     Quart.  Joum.  Geol.  Soc.  1865,  vol.  xxii.  p.  9. 
Sir  John  Lubbock. — On  the  Parallel  Roads  of  Glen  Roy.    Quart. 

Joum.  Geol.  Soc.  1867,  vol.  xxiv.  p.  83. 
Charles  Babbage. — Observations  on  the  Parallel  Roads  of  Glen 

Roy.     Quart.  Joum.  Geol.  Soc.  1868,  vol.  xxiv.  p.  273. 
James  Nicol. — On  the  Origin  of  the  Parallel  Roads  of  Glen  Roy. 

1869.    Geol.  Soc.  Journal,  vol.  xxv.  p.  282. 
James  Nicol. — How  the  Parallel  Roads  of  Glen  Roy  were  formed. 

1872.    Geol.  Soc.  Journal,  vol.  xxviii.  p.  237. 
Major-General  Sir  Henry  James,  R.  E..— Notes  on  the  Parallel 

Roads  of  Lochaber.    4to.     1874. 


IX. 
ALPINE  SCULPTURE. 

1864. 

TO  account  for  the  conformation  of  the  Alps,  two 
hypotheses  have  been  advanced,  which  may  be 
respectively  named  the  hypothesis  of  fracture  and  the 
hypothesis  of  erosion.  The  former  assumes  that  the 
forces  by  which  the  mountains  were  elevated  produced 
fissures  in  the  earth's  crust,  and  that  the  valleys  of  the 
Alps  are  the  tracks  of  these  fissures  ;  while  the  latter 
maintains  that  the  valleys  have  been  cut  out  b^f  the 
action  of  ice  and  water,  the  mountains  themselves  being 
the  residual  forms  of  this  grand  sculpture.  I  had  heard 
the  Via  Mala  cited  as  a  conspicuous  illustration  of  the 
fissure  theory — the  profound  chasm  thus  named,  and 
through  which  the  Hinter-Rhein  now  flows,  could,  it 
was  alleged^  be  nothing  else  than  a  crack  in  the  earth's 
crust.  To  the  Via  Mala  I  therefore  went  in  1864  to 
instruct  myself  upon  the  point  in  question. 

The  gorge  commences  about  a  quarter  of  an  hour 
above  Tusis ;  and,  on  entering  it,  the  first  impression 
certainly  is  that  it  must  be  a  fissure.  This  conclusion 
in  my  case  was  modified  as  I  advanced.  Some  distance 
up  the  gorge  I  found  upon  the  slopes  to  my  right 
quantities  of  rolled  stones,  evidently  rounded  by  water- 
action.  Still  further  up,  and  just  before  reaching  the 
first  bridge  which  spans  the  chasm,  I  foimd  more  rolled 


230  FEAGMENTS  OF  SCIENCE. 

stones,  associated  with  sand  and  gravel.  Through  this 
mass  of  detritus,  fortunately,  a  vertical  cutting  had 
been  made,  which  exhibited  a  section  showing  perfect 
stratification.  There  was  no  agency  in  the  place  to  roll 
these  stones,  and  to  deposit  these  alternating  layers  of 
sand  and  pebbles,  but  the  river  which  now  rushes  some 
hundreds  of  feet  below  them.  At  one  period  of  the  Via 
Mala's  history  the  river  must  have  run  at  this  high 
level.  Other  evidences  of  water-action  soon  revealed 
themselves.  From  the  parapet  of  the  first  bridge  I 
could  see  the  solid  rock  200  feet  above  the  bed  of  the 
river  scooped  and  eroded. 

It  is  stated  in  the  guide-books  that  the  river,  which 
usually  runs  along  the  bottom  of  the  goi-ge,  has  been 
known  almost  to  fill  it  during  violent  thunder-storms;  and 
it  may  be  urged  that  the  marks  of  erosion  which  the  sides 
of  the  chasm  exhibit  are  due  to  those  occasional  floods. 
In  reply  to  this,  it  may  be  stated  that  even  the  exi  t- 
ence  of  such  floods  is  not  well  authenticated,  and  that 
if  the  supposition  w^-re  true,  it  would  be  an  additional 
argument  in  favour  of  the  cutting  power  of  the  river. 
For  if  floods  operating  at  rare  intervals  could  thus 
erode  the  rock,  the  same  agency,  acting  without  ceasing 
upon  the  river's  bed,  must  certainly  be  competent  to 
excavate  it. 

I  proceeded  upwards,  and  from  a  point  near  another 
bridge  (which  of  them  I  did  not  note)  had  a  fine  view 
of  a  portion  of  the  gorge.  The  river  here  runs  at  the 
bottom  of  a  cleft  of  profound  depth,  but  so  narrow  that 
it  might  be  leaped  across.  That  tins  cleft  must  be  a 
crack  is  the  impression  first  produci  d  ;  but  a  brief  in- 
spection suffices  to  prove  that  it  has  been  cut  by  the 
river.  From  top  to  bottom  we  have  the  unmistakable 
marks  of  erosion.  This  cleft  was  best  seen  on  looking 
downwards  from  a  point  near  the  bridge  ;  but  looking 


ALPINE  SCULPTUKE.  231 

upwards  from  the  bridge  itself,  the  evidence  of  aqueous 
erosion  was  equally  convincing. 

The  character  of  the  erosion  depends  upon  the  rock 
as  well  as  upon  the  river.  The  action  of  water  upon 
some  rocks  is  almost  purely  mechanical ;  they  are 
simply  groimd  away  or  detached  in  sensible  masses^ 
Water,  however,  in  passing  over  limestone,  charges  it 
self  with  cnrbonate  of  lime  without  damage  to  its  trans 
parency  ;  the  rock  is  dissolved  in  the  water  ;  and  the 
gorges  cut  by  water  in  such  rocks  often  re  cmble  those 
cut  in  the  ice  of  glaciers  by  glacier  streams.  To  the 
solubility  of  limestone  is  probably  to  be  ascribed  the 
fantastic  forms  which  peaks  of  this  rock  usually  assume, 
and  also  the  grottos  and  caverns  which  interpenetrate 
limestone  formations.  A  rock  capable  of  being  thus 
dissolved  will  expose  a  smooth  surface  after  the  water 
has  quitted  it ;  and  in  the  case  of  the  Via  Mala  it  is  the 
polish  of  the  surfaces  and  the  curved  hollows  scooped  in 
the  sides  of  the  gorge,  which  assure  us  that  the  chasm 
has  been  the  work  of  the  river. 

About  four  miles  from  Tusis,  and  not  far  from  the 
little  village  of  Zillis,  the  Via  Mala  opens  into  a  plain 
bounded  by  high  terraces.  It  occurred  to  me  the 
moment  I  saw  it  that  the  plain  had  been  the  bed  of  an 
ancient  lake  ;  and  a  farmer,  who  was  my  temporary 
companion,  immediately  informed  me  that  such  was 
the  tradition  of  the  neighbourhood.  This  man  con- 
versed with  intelligence,  and  as  I  drew  his  attention  to 
the  rolled  stones,  which  rest  not  only  above  the  river, 
but  above  the  road,  and  inferred  that  the  river  must 
once  have  been  there  to  have  rolled  those  stones,  he  saw 
the  force  of  the  evidence  perfectly.  In  fact,  in  former 
times,  and  subsequent  to  the  retreat  of  the  great 
glaciers,  a  rocky  barrier  crossed  the  valley  at  this  place, 
damming   the  river   which  came  from  the  mountains 

16 


232  FRAGMENTS  OF  SCIENCE. 

higher  up.  A  lake  was  thus  formed  which  poured  its 
waters  over  the  barrier.  Two  actions  were  here  at  work, 
both  tending  to  obliterate  the  lake — the  raising  of  its 
bed  by  the  deposition  of  detritus,  and  the  cutting  of  its 
dam  by  the  river.  In  process  of  time  the  cut  deepened 
into  the  Via  Mala ;  the  lake  was  drained,  and  the  river 
now  flows  in  a  definite  channel  through  the  plain  which 
its  waters  once  totally  covered. 

From  Tusis  I  crossed  to  Tiefenkasten  by  the  Schien 
Pass,  and  thence  over  the  Julier  Pass  to  Pontresina. 
There  are  three  or  four  ancient  lake-beds  between  Tiefen- 
kasten and  the  summit  of  the  Julier.  They  are  all  of 
the  same  type — a  more  or  less  broad  and  level  valley- 
bottom,  with  a  barrier  in  front  through  which  the 
river  has  cut  a  passage,  the  drainage  of  the  lake  being 
the  consequence.  These  lakes  were  sometimes  dammed 
by  barriers  of  rock,  sometimes  by  the  moraines  of 
ancient  glaciers. 

An  example  of  this  latter  kind  occurs  in  the  Rosegg 
valley,  about  twenty  minutes  below  the  end  of  the 
Eosegg  glacier,  and  about  an  hour  from  JPontresina. 
The  valley  here  is  crossed  by  a  pine-covered  moraine 
of  the  noblest  dimensions;  in  the  neighbourhood  of 
London  it  mi^ht  be  called  a  mountain.  That  it  is  a 
moraine,  the  inspection  of  it  from  a  point  on  the  Surlei 
slopes  above  it  will  convince  any  person  possessing  an 
educated  eye.  Where,  moreover,  the  interior  of  the 
mound  is  exposed,  it  exhibits  moraine-matter — detritus 
pulverised  by  the  ice,  with  boulders  entangled  in  it. 
It  stretched  quite  across  the  valley,  and  at  one  time 
dammed  the  river  up.  But  now  the  barrier  is  cut 
through,  the  stream  having  about  one-fourth  of  the 
moraine  to  its  right,  and  the  remaining  three-fourths 
to  its  left.  Other  moraines  of  a  more  resisting  charac- 
ter hold  their  ground  as  barriers  to  the  present  day. 


ALPINE  SCULPTUKE.  233 

In  the  Val  di  Campo,  for  example,  about  three-quarters 
of  an  hour  from  Pisciadello,  there  is  a  moraine  com- 
posed of  large  boulders,  which  interrupt  the  course  of  a 
river  and  compel  the  water  to  fall  over  them  in  cascades. 
They  have  in  great  part  resisted  its  action  since  the 
retreat  of  the  ancient  glacier  which  formed  the  moraine. 
Behind  the  moraine  is  a  lake-bed,  now  converted  into  a 
level  meadow,  which  rests  on  a  deep  layer  of  mould. 

At  Pontresina  a  very  fine  and  instructive  gorge  is  to 
be  seen.  The  river  from  the  Morteratsch  glacier  rushes 
through  a  deep  and  narrow  chasm  which  is  spanned  at 
one  place  by  a  stone  bridge.  The  rock  is  not  of  a  charac- 
ter to  preserve  smooth  polishing ;  but  the  larger  features 
of  water-action  are  perfectly  evident  from  top  to  bottom. 
Those  features  are  in  part  visible  from  the  bridge,  but 
still  better  from  a  point  a  little  distance  from  the 
bridge  in  the  direction  of  the  upper  village  of  Pont- 
resina. The  hollowing  out  of  the  rock  by  the  eddies  of 
the  water  is  here  quite  manifest.  A  few  minutes'  walk 
upwards  brings  us  to  the  end  of  the  gorge  ;  and  behind 
it  we  have  the  usual  indications  of  an  ancient  lake,  and 
terraces  of  distinct  water  origin.  From  this  position 
indeed  the  genesis  of  the  gorge  is  clearly  revealed. 
After  the  retreat  of  the  ancient  glacier,  a  transverse 
ridge  of  comparatively  resisting  material  crossed  the 
valley  at  this  place.  Over  the  lowest  part  of  this  ridge 
the  river  flowed,  rushing  steeply  dovm  to  join  at  the 
bottom  of  the  slope  the  stream  which  issued  from  the 
Rosegg  glacier.  On  this  incline  the  water  became  a 
])owerful  eroding  agent,  and  finally  cut  the  channel  to 
its  present  depth. 

Geological  writers  of  reputation  assume  at  this 
place  the  existence  of  a  fissure,  the  '  washing  out '  of 
which  resulted  in  the  formation  of  the  gorge.  Now 
no  examination  of  the  bed  of  the  river  ever  proved  the 


234  FRAGMENTS  OF  SCIENCE. 

existence  of  this  fissure  ;  and  it  is  certain  that  water, 
particularly  when  charged  with  solid  matter  in  suspen- 
sion, can  cut  a  channel  through  unfissured  rock.  Cases 
of  deep  cutting  can  be  pointed  out  where  the  clean  bed 
of  the  stream  is  exposed,  the  rock  wliich  forms  the 
floor  of  the  river  not  exhibiting  a  trace  of  fissure.  An 
example  of  this  kind  on  a  small  scale  occurs  near  the 
Bernina  Grasthaus,  about  two  hours  from  Pontresina.  A 
little  way  below  the  junction  of  the  two  streams  from  the 
Bernina  Pass  and  the  Heutlial  the  river  flows  through  a 
channel  cut  by  itself,  and  20  or  30  feet  in  depth.  At  some 
places  the  river-bed  is  covered  with  rolled  stones ;  at 
other  places  it  is  bare,  but  shows  no  trace  of  fissure.  The 
abstract  power  of  water,  if  I  may  use  the  term,  to  cut 
through  rock  is  demonstrated  by  such  instances.  But  if 
water  be  competent  to  form  a  gorge  without  the  aid  of 
a  fissure,  why  assume  the  existence  of  such  fissures  in 
cases  like  that  at  Pontresina  ?  It  seems  far  more 
philosophical  to  accept  the  simple  and  impressive  history 
written  on  the  walls  of  those  gorges  by  the  agent  which 
produced  them. 

Numerous  cases  might  be  pointed  out,  varying  in 
magnitude,  but  all  identical  in  kind,  of  barriers  which 
crossed  valleys  and  formed  lakes  having  been  cut  through 
by  rivers,  narrow  gorges  being  the  consequence.  One 
of  the  most  famous  examples  of  this  kind  is  the  Finster- 
aarschlucht  in  the  valley  of  Hasli.  Here  the  ridge  called 
the  Kirchet  seems  split  across,  and  the  river  Aar  rushes 
through  the  fissure.  Behind  the  barrier  we  have  the 
meadows  and  pastures  of  Imhof  resting  on  the  sediment 
of  an  ancient  lake.  Were  this  an  isolated  case,  one 
might  with  an  apparent  show  of  reason  conclude  that  the 
Finsteraarschlucht  was  produced  by  an  earthquake,  as 
Home  suppose  it  to  have  been  ;  but  when  we  find  it  to  be 
a  single  sample  of  actions  which  are  frequent  in  the  Alps 


ALPINE  SCULPTUEE.  235 

' — ^when  probably  a  hundred  cases  of  the  same  kind,  though 
different  in  magnitude,  can  be  pointed  out — it  seems 
quite  unphilosophical  to  assume  that  in  each  particular 
case  an  earthquake  was  at  hand  to  form  a  channel  for  the 
river.  As  in  the  case  of  the  barrier  at  Pontresina,  the 
Kirchet,  after  the  retreat  of  the  Aar  glacier,  dammed  the 
waters  flowing  from  it,  thus  forming  a  lake,  on  the  bed 
of  whicli  now  stands  the  village  of  Imhof.  Over  this 
barrier  the  x\ar  tumbled  towards  Meyringen,  cutting,  as 
the  centuries  passed,  its  bed  ever  deeper,  until  finally 
it  became  deep  enough  to  drain  the  lake,  leaving  in  its 
place  the  alluvial  plain,  through  which  the  river  now 
flows  in  a  definite  channel. 

In  1866 1  subjected  the  Finsteraarschlucht  to  a  close 
examination.  The  earthquake  theory  already  adverted 
to  was  then  prevalent  regarding  it,  and  I  wished  to  see 
whether  any  evidences  existed  of  aqueous  erosion.  Near 
the  summit  of  the  Kirchet  is  a  signboard  inviting 
the  traveller  to  visit  the  Aarensckiucht,  a  narrow 
lateral  gorge  which  runs  down  to  the  very  bottom  of 
the  principal  one.  The  aspect  of  this  smaller  chasm 
from  bottom  to  top  proves  to  demonstration  that  water 
had  in  former  ages  been  there  at  work.  It  is  scooped, 
rounded,  and  polished,  so  as  to  render  palpable  to 
the  most  careless  eye  that  it  is  a  gorge  of  erosion. 
But  it  was  regarding  the  sides  of  the  great  chasm  that 
instruction  was  needed,  and  from  its  edge  nothing  to 
satisfy  me  could  be  seen.  I  therefore  stripped  and  waded 
into  the  river  until  a  point  was  reached  which  com- 
manded an  excellent  view  of  both  sides  of  the  gorge. 
The  water  was  cutting  cold,  but  I  was  repaid.  Below 
me  on  the  left-hand  side  was  a  jutting  cliff  which  bore 
the  thrust  of  the  river  and  caused  the  Aar  to  swerve 
from  its  direct  course.  From  top  to  bottom  this  cliff 
was  polished,  rounded,  and  scooped.     There  was   no 


236  FRAHMENTS  OF  SCIENCE. 

room  for  doubt.  The  river  which  now  runs  so  deeply 
down  had  once  been  above.  It  has  been  the  delver  of 
its  own  channel  through  the  barrier  of  the  Kirchet. 

But  the  broad  view  taken  by  the  advocates  of  the 
fracture  theory  is,  that  the  valleys  themselves  follow 
the  tracks  of  primeval  fissures  produced  by  the  upheaval 
of  the  land,  the  cracks  across  the  barriers  referred  to 
being  in  reality  portions  of  the  great  cracks  which 
formed  the  valleys.  Such  an  argument,  however,  would 
virtually  concede  the  theory  of  erosion  as  applied  to  the. 
valleys  of  the  Alps.  The  narrow  gorges,  often  not  more 
than  twenty  or  thirty  feet  across,  sometimes  even 
narrower,  frequently  occur  at  the  bottom  of  broad 
valleys.  Such  fissures  might  enter  into  the  list  of 
accidents  which  gave  direction  to  the  real  erosive  agents 
which  scooped  the  valley  out ;  but  the  formation  of  the 
valley,  as  it  now  exists,  could  no  more  be  ascribed  to 
such  cracks  than  the  motion  of  a  railway  train  could 
be  ascribed  to  the  finger  of  the  engineer  which  turns  on 
the  steam. 

These  deep  gorges  occur,  I  believe,  for  the  most 
part  in  limestone  strata  ;  and  the  effects  which  the 
merest  driblet  of  water  can  produce  on  limestone  are 
quite  astonishing.  It  is  not  uncommon  to  meet  chasms 
of  considerable  depth  produced  by  small  streams  the 
beds  of  which  are  dry  for  a  large  portion  of  the  year. 
Eight  and  left  of  the  larger  gorges  such  secondary 
chasms  are  often  found.  The  idea  of  time  must,  I 
think,  be  more  and  more  included  in  our  reasonings  on 
these  phenomena.  Happily,  the  marks  which  the  rivers 
have,  in  most  cases,  left  behind  them,  and  which  refer, 
geologically  considered,  to  actions  of  yesterday,  give  us 
ground  and  courage  to  conceive  what  maybe  effected  in 
geologic  periods.  Thus  the  modern  portion  of  the  Via 
Mala  throws  light  upon  the  whole.  Near  Bergiin,  in 
the  valley  of  the  Albula,  there  is  also  a  Little  Via  INIala, 


ALPINE  SCULPTURE.  237 

which  is  not  less  significant  than  the  great  one.  The 
river  flows  here  through  a  profound  limestone  gorge, 
and  to  the  very  edges  of  the  gorge  we  have  the  evidences 
of  erosion.  But  the  most  striking  illustration  of  water- 
action  upon  limestone  rock  that  I  have  ever  seen  is 
the  gorge  at  Pfatiers.  Here  the  traveller  passes 
along  the  side  of  the  chasm  midway  between  top  and 
bottom.  Whichever  way  he  looks,  backwards  or  for- 
wards, upwards  or  downwards,  towards  the  sky  or  to- 
wards the  river,  he  meets  everywhere  the  irresistible 
and  impressive  evidence  that  this  wonderful  fissure  has 
been  sawn  through  the  mountain  by  the  waters  of  the 
Tamina. 

I  have  thus  far  confined  myself  to  the  consideration 
of  the  gorges  formed  by  the  cutting  tlu-ough  of  the  rock- 
barriers  which  frequently  cross  the  valleys  of  the  Alps ; 
as  far  as  they  have  been  examined  by  me  they  are  the 
work  of  erosion.  But  the  larger  question  still  remains.  To 
what  action  are  we  to  ascribe  the  formation  of  the  valleys 
themselves  ?  This  question  includes  that  of  the  forma- 
tion of  the  mountain-ridges,  for  were  the  valleys  wholly 
filled,  the  ridges  would  disappear.  Possibly  no  answer 
can  be  given  to  this  question  which  is  not  beset  with  more 
or  less  of  difficulty.  Special  localities  might  be  found 
which  would  seem  to  contradict  every  solution  which 
refers  the  conformation  of  the  Alps  to  the  operation  of 
a  single  cause. 

StiU  the  Alps  present  features  of  a  character  suffi- 
ciently definite  to  bring  the  question  of  their  origin  with- 
in the  sphere  of  close  reasoning.  That  they  were  in  whole 
or  in  part  once  beneath  the  sea  will  not  be  disputed ;  for 
they  are  in  great  part  composed  of  sedimentary  rocks 
which  required  a  sea  to  form  them.  Their  present  elevation 
above  the  sea  is  due  to  one  of  those  local  changes  in  the 
shape  of  the  earth  which  have  been  of  frequent  occurrence 


238  FEAGMENTS  OF  SCIENCE. 

throughout  geologic  time,  in  some  cases  depressing  the 
land,  and  in  others  causing  the  sea-bottom  to  protrude 
beyond  its  surface.  Considering  the  inelastic  character  of 
its  materials,  the  protuberance  of  the  Alps  could  hardly 
have  been  pushed  out  without  dislocation  and  fracture ; 
and  this  conclusion  gains  in  probability  when  we  con- 
sider the  foldings,  contortions,  and  even  reversals  in 
position  of  the  strata  in  many  parts  of  the  Alps. 
Such  changes  in  the  position  of  beds  which  were  once 
horizontal  could  not  have  been  effected  without  disloca- 
tion. Fissures  would  be  produced  by  these  changes ; 
and  such  fissures,  the  advocates  of  the  fracture  theory 
contend,  mark  the  positions  of  the  valleys  of  the  Alps. 

Imagination  is  necessary  to  the  man  of  science,  and 
we  could  not  reason  on  our  present  subject  without  the 
power  of  presenting  mentally  a  picture  of  the  earth's 
crust  cracked  and  fissured  by  the  forces  wliich  produced 
its  upheaval.  Imagination,  however,  must  be  strictly 
checked  by  reason  and  by  observation.  That  fractures 
occurred  cannot,  I  think,  be  doubted,  but  that  the  valleys 
of  the  Alps  are  thus  formed  is  a  conclusion  not  at  all 
involved  in  the  admission  of  dislocations.  I  never  met 
with  a  precise  statement  of  the  manner  in  which  the 
advocates  of  the  fissure  theory  suppose  the  forces  to  have 
acted — whether  they  assume  a  general  elevation  of  the 
region,  or  a  local  elevation  of  distinct  ridges  ;  or  whether 
they  assume  local  subsidences  after  a  general  elevation, 
or  whether  they  would  superpose  upon  the  general  up- 
heaval minor  and  local  upheavals. 

In  the  absence  of  any  distinct  statement,  I  will 
assume  the  elevation  to  be  general — that  a  swelling  out 
of  the  earth's  crust  occurred  here,  sufficient  to  place  the 
most  prominent  portions  of  the  protuberance  three  miles 
above  the  sea-level.  To  fix  the  ideas,  let  us  consider  a 
circular  portion  of  the  crust,  say  one  hundred   miles  in 


ALPINE  SCULPTUEE.  239 

diameter,  and  let  us  suppose,  in  the  first  instance,  the 
circumference  of  this  circle  to  remain  fixed,  and  that 
the  elevation  was  confined  to  the  space  within  it.  The 
upheaval  would  throw  the  crust  into  a  state  of  strain; 
and,  if  it  were  inflexible,  the  strain  must  be  relieved  by 
fracture.  Crevasses  would  thus  intersect  the  crust.  Let 
us  now  enquire  what  proportion  the  area  of  these  open 
fissures  is  likely  to  bear  to  the  area  of  the  unfissured 
crust.  An  approximate  answer  is  all  that  is  here  re- 
quired ;  for  the  problem  is  of  such  a  character  as  to 
lender  minute  precision  unnecessary. 

No  one,  I  think,  would  affirm  that  the  area  of  the 
fissures  would  be  one-hundredth  the  area  of  the  land. 
For  let  us  consider  the  strain  upon  a  single  line  drawn 
over  the  summit  of  the  protuberance  from  a  point  on 
its  rim  to  a  point  opposite.  Eegarding  the  protuberance 
as  a  spherical  swelling,  the  length  of  the  arc  corre- 
sponding to  a  chord  of  100  miles  and  a  versed  sine 
of  3  miles  is  100*24  miles  ;  consequently  the  surface  to 
reach  its  new  position  must  stretch  0*24  of  a  mile,  or 
be  broken.  A  fissure  or  a  nurnber  of  cracks  with  this 
total  width  would  relieve  the  strain  ;  that  is  to  say,  the 
sum  of  the  widths  of  all  the  cracks  over  the  length  of 
100  miles  would  be  420  yards.  If,  instead  of  com- 
paring the  width  of  the  fissures  with  the  length  of  the 
lines  of  tension,  we  compared  their  areas  with  the  area 
of  the  unfissured  land,  we  should  of  course  find  the 
proportion  much  less.  These  considerations  will  help 
the  imagination  to  realise  what  a  small  ratio  the  area 
of  the  open  fissures  must  bear  to  the  unfissured  crust. 
Tbey  enable  us  to  say,  for  example,  that  to  assume  the 
area  of  the  fissures  to  be  one-tenth  of  the  area  of  the 
land  would  be  quite  absurd,  while  that  the  area  of  the 
fissures  could  be  one-half  or  more  than  one-half  that  of 
the  land  would  be  in  a  proportionate  degree  unthink- 


240  FRAGMENTS  OF  SCIENCE. 

able.  If  we  suppose  the  elevation  to  be  due  to  the 
shrinking  or  subsidence  of  the  land  all  round  our 
assumed  circle,  we  arrive  equally  at  the  conclusion 
that  the  area  of  the  open  fissures  would  be  altogether 
insignificant  as  compared  with  that  of  the  unfissured 
crust. 

To  those  who  have  seen  them  from  a  commanding 
elevation,  it  is  needless  to  say  that  the  Alps  themselves 
bear  no  sort  of  resemblance  to  the  picture  which  this 
theory  presents  to  us.  Instead  of  deep  cracks  with 
approximately  vertical  walls,  we  have  ridges  running 
into  peaks,  and  gradually  sloping  to  form  valleys. 
Instead  of  a  fissured  crust,  we  have  a  state  of  things 
closely  resembling  the  surface  of  the  ocean  when  agi- 
tated by  a  storm.  The  valleys,  instead  of  being  much 
narrower  than  the  ridges,  occupy  the  greater  space.  A 
plaster  cast  of  the  Alps  turned  upside  down,  so  as  to 
invert  the  elevations  and  depressions,  would  exhibit 
blunter  and  broader  mountains,  with  narrower  valleys 
between  them,  than  the  present  ones.  The  valleys  that 
exist  cannot,  I  think,  with  any  correctness  of  language 
be  called  fissures.  It  may  be  urged  that  they  origi- 
nated in  fissures :  but  even  this  is  unproved,  and,  were 
it  proved,  the  fissures  would  still  play  the  subordinate 
part  of  giving  direction  to  the  agents  which  are  to  be 
regarded  as  the  real  sculptors  of  the  Alps. 

The  fracture  theory,  then,  if  it  regards  the  elevation 
of  the  Alps  as  due  to  the  operation  of  a  force  acting 
throughout  the  entire  region,  is,  in  my  opinion,  utterly 
incompetent  to  account  for  the  conformation  of  the 
country.  If,  on  the  other  hand,  we  are  compelled  to 
resort  to  local  disturbances,  the  manipulation  of  the 
earth's  crust  necessary  to  obtain  the  valleys  and  the 
mountains  will,  I  imagine,  bring  the  difficulties  of  the 
theory  into  very  strong  relief.     Indeed  an  examination 


ALPINE  SCULPTUKE.  241 

of  the  region  from  many  of  the  more  accessible  emi- 
nences— from  the  Gralenstock,  the  Grauhaupt,  the  Pitz 
Lauguard,  the  Monte  Confinaie —  or,  better  still,  from 
Mont  Blanc,  Monte  Eosa,  the  Jungfrau,  the  Finsteraar- 
horn,  the  Weisshorn,  or  the  Matterhorn,  where  local 
peculiarities  are  toned  down,  and  the  operations  of  the 
powers  which  really  made  this  region  what  it  is  are 
alone  brought  into  prominence — must,  I  imagine,  con- 
vince every  physical  geologist  of  the  inability  of  any 
fracture  theory  to  account  for  the  present  conformation 
of  the  Alps. 

A  correct  model  of  the  mountains,  with  an  un- 
exaggerated  vertical  scale,  produces  the  same  effect 
upon  the  mind  as  the  prospect  from  one  of  the  highest 
peaks.  We  are  apt  to  be  influenced  by  local  phenomena 
which,  though  insignificant  in  view  of  the  general 
question  of  Alpine  conformation,  are,  with  reference  to 
our  customary  standards,  vast  and  impressive.  In  a 
true  model  those  local  peculiarities  disappear ;  for  on 
the  scale  of  a  model  they  are  too  small  to  be  visible ; 
while  the  essential  facts  and  forms  are  presented  to  the 
undistracted  attention. 

A  minute  analysis  of  the  phenomena  strengthens 
the  conviction  which  the  general  aspect  of  the  Alps 
fixes  in  the  mind.  We  find,  for  example,  numerous 
valleys  which  the  most  ardent  plutonist  would  not 
think  of  ascribing  to  any  other  agency  than  erosion. 
That  such  is  their  genesis  and  history  is  as  certain  as 
that  erosion  produced  the  Chines  in  the  Isle  of  Wight. 
From  these  indubitable  cases  of  erosion — commencing, 
if  necessary,  with  the  small  ravines  which  run  down 
the  flanks  of  the  ridges,  with  their  little  working 
navigators  at  their  bottoms — we  can  proceed,  by  almost 
insensible  gradations,  to  the  largest  valleys  of  the 
Alps ;  and  it  would  perplex  the  plutonist  to  fix  upon 


242  FRAGMENTS  OF  SCIENCE. 

the  point  at  which  fracture  begins  to  play  a  material 
part* 

In  ascending  one  of  the  larger  valleys,  we  enter  it 
where  it  is  wide  and  where  the  eminences  are  gentle  on 
either  side.  The  flanking  mountains  become  higher 
and  more  abrupt  as  we  ascend,  and  at  length  we  reach 
a  place  where  the  depth  of  the  valley  is  a  maximum. 
Continuing  our  walk  upwards,  we  find  ourselves  flanked 
by  gentler  slopes,  and  finally  emerge  from  the  valley 
and  reach  the  summit  of  an  open  col,  or  depression  in 
the  chain  of  mountains.  This  is  the  common  character 
of  the  large  valleys.  Crossing  the  col,  we  descend 
along  the  opposite  slope  of  the  chain,  and  through  the 
same  series  of  appearances  in  the  reverse  order.  If  the 
valleys  on  both  sides  of  the  col  were  produced  by 
fissures,  what  prevents  the  fissure  from  prolonging 
itself  across  the  col?  The  case  here  cited  is  repre- 
sentative ;  and  I  am  not  acquainted  with  a  single 
instance  in  the  Alps  where  the  chain  has  been  cracked 
in  the  manner  indicated.  The  cols  are  simply  de- 
pressions ;  in  many  of  which  the  unfissured  rock  can 
be  traced  from  side  to  side. 

The  typical  instance  just  sketched  follows  as  a 
natural  consequence  from  the  theory  of  erosion.  Before 
either  ice  or  water  can  exert  great  power  as  an  erosive 
agent,  it  must  collect  in  sufificient  mass.  On  the  higher 
slopes  and  plateaus — in  the  region  of  cols — the  power 
is  not  fully  developed ;  but  lower  down  tributaries 
unite,  erosion  is  carried  on  with  increased  vigour,  and 
the  excavation  gradually  reaches  a  maximum.  Lower 
still  the  elevations  diminish  and  the  slopes  become 
more  gentle;  the  cutting  power  gradually  relaxes, 
imtil  finally  the  eroding  agent  quits  the  mountains 
altogether,  and  the  gj-and  effects  which  it  produced  in 
the  earlier  portions  of  its  course  entirely  disappear. 


ALPINE  SCULPTURE.  243 

I  have  hitherto  confined  myself  to  the  consideration 
of  the  broad  question  of  the  erosion  theory  as  compared 
with  the  fracture  theory ;  and  all  that  I  have  been  able 
to  observe  and  think  with  reference  to  the  subject 
leads  me  to  adopt  the  former.  Under  the  term  erosion 
I  include  the  action  of  water,  of  ice,  and  of  the  atmo- 
sphere, including  frost  and  rain.  Water  and  ice, 
however,  are  the  principal  agents,  and  which  of  these 
two  has  produced  the  greatest  effect  it  is  perhaps  im- 
possible to  say.  Two  years  ago  I  wrote  a  brief  note 
'  On  the  Conformation  of  the  Alps,'  ^  in  which  I  ascribed 
the  paramount  influence  to  glaciers.  The  facts  on 
which  that  opinion  was  founded  are,  I  think,  un- 
assailable ;  but  whether  the  conclusion  then  announced 
fairly  follows  from  the  facts  is,  I  confess,  an  open 
question. 

The  arguments  which  have  been  thus  far  urged 
against  the  conclusion  are  not  convincing.  Indeed, 
the  idea  of  glacier  erosion  appears  so  daring  to  some 
minds  that  its  boldness  alone  is  deemed  its  sufficient 
refutation.  It  is,  however,  to  be  remembered  that  a 
precisely  similar  position  was  taken  up  by  many  ex- 
cellent workers  when  the  question  of  ancient  glacier 
extension  was  first  mooted.  The  idea  was  considered 
too  hardy  to  be  entertained ;  and  the  evidences  of 
glacial  action  were  sought  to  be  explained  by  reference 
to  almost  any  process  rather  than  the  true  one.  Let 
those  who  so  wisely  took  the  side  of  '  boldness '  in  that 
discussion  beware  lest  they  place  themselves,  with 
reference  to  the  question  of  glacier  erosion,  in  the 
position  formerly  occupied  by  their  opponents. 

Looking  at  the  little  glaciers  of  the  present  day — 
mere  pigmies  as  compared  to  the  giants  of  the  glacial 
epoch — we  find  that  from  every  one  of  them  issues  a 
•  Phil.  Mag.  vol.  xxiv.  p.  IQd, 


244  FRAGMENTS  OF  SCIENCE. 

river  more  or  less  voluminous,  charged  with  the  matter 
which  the  ice  has  rubbed  from  the  rocks.  Where  the 
rocks  are  soft,  the  amount  of  this  finely  pulverised 
matter  suspended  in  the  water  is  very  great.  The 
water,  for  example,  of  the  river  which  flows  from 
Santa  Catarina  to  Bormio  is  thick  with  it.  The  Ehine 
is  charged  with  this  matter,  and  by  it  has  so  silted  up 
the  Lake  of  Constance  as  to  abolish  it  for  a  large 
fraction  of  its  length.  The  Khone  is  charged  with  it, 
and  tens  of  thousands  of  acres  of  cultivable  land  are 
formed  by  the  silt  above  the  Lake  of  Greneva. 

In  the  case  of  every  glacier  we  have  two  agents  at 
work — the  ice  exerting  a  crushing  force  on  every  point 
of  its  bed  which  bears  its  weight,  and  either  rasping 
this  point  into  powder  or  teaming  it  bodily  from  the 
rock  to  which  it  belongs  ;  while  the  water  which  every- 
where circulates  upon  the  bed  of  the  glacier  continually 
washes  the  detritus  away  and  leaves  the  rock  clean  for 
further  abrasion.  Confining  the  action  of  glaciers  to 
the  simple  rubbing  away  of  the  rocks,  and  allowing 
them  sufficient  time  to  act,  it  is  not  a  matter  of  opinion, 
but  a  physical  certainty,  that  they  will  scoop  out  valleys. 
But  the  glacier  does  more  than  abrade.  Eocks  are  not 
homogeneous ;  they  are  intersected  by  joints  and  places 
of  weakness,  which  divide  them  into  virtually  detached 
masses.  A  glacier  is  undoubtedly  competent  to  root 
such  masses  bodily  away.  Indeed  the  mere  a  priori 
consideration  of  the  subject  proves  the  competence  of  a 
glacier  to  deepen  its  bed.  Taking  the  case  of  a  glacier 
1,000  feet  deep  (and  some  of  the  older  ones  were 
probably  three  times  this  depth),  and  allowing  40  feet 
of  ice  to  an  atmosphere,  we  find  that  on  every  square 
inch  of  its  bed  such  a  glacier  presses  with  a  weight  of 
375  lbs.,  and  on  every  square  yard  of  its  bed  with  a 
weight  of  486,000  lbs.     With  a  vertical  pressure  of 


ALPINE  SCULPTURE.  245 

this  amount  the  glacier  is  urged  down  its  valley  by  the 
pressure  from  behind.  We  can  hardly,  I  think,  deny 
to  such  a  tool  a  power  of  excavation. 

The  retardation  of  a  glacier  by  its  bed  has  been 
referred  to  as  proving  its  impotence  as  an  erosive 
agent ;  but  this  very  retardation  is  in  some  measure  an 
expression  of  the  magnitude  of  the  erosive  energy. 
Either  the  bed  must  give  way,  or  the  ice  must  slide 
over  itself.  We  get  indeed  some  idea  of  the  crushing 
pressure  which  the  moving  glacier  exercises  against  its 
bed  from  the  fact  that  the  resistance,  and  the  effort  to 
overcome  it,  are  such  as  to  make  the  upper  layers  of  a 
glacier  move  bodily  over  the  lower  ones — a  portion 
only  of  the  total  motion  being  due  to  the  progress  of 
the  entire  mass  of  the  glacier  down  its  valley. 

The  sudden  bend  in  the  valley  of  the  Khone  at 
Martigny  has  also  been  regarded  as  conclusive  evidence 
against  the  theory  of  erosion.  'Why,'  it  has  been 
asked,  '  did  not  the  glacier  of  the  Ehone  go  straight 
forward  instead  of  making  this  awkward  bend  ? '  But 
if  the  valley  be  a  crack,  why  did  the  crack  make  this 
bend?  The  crack,  I  submit,  had  at  least  as  much 
reason  to  prolong  itself  in  a  straight  line  as  the  glacier 
had.  A  statement  of  Sir  John  Herschel  with  reference 
to  another  matter  is  perfectly  applicable  here :  '  A 
crack  once  produced  has  a  tendency  to  run — for  this 
plain  reason,  that  at  its  momentary  limit,  at  the  point 
at  which  it  has  just  arrived,  the  divellent  force  on  the 
molecules  there  situated  is  counteracted  only  by  half 
of  the  cohesive  force  which  acted  when  there  was  no 
crack,  viz.  the  cohesion  of  the  uncracked  portion  alone  * 
(*Proc.  Eoy.  Soc'  vol.  xii.  p.  678).  To  account,  then, 
for  the  bend,  the  adherent  of  the  fracture  theory  must 
assume  the  existence  of  some  accident  which  turned 
the  crack  at  right  angles  to  itself ;  and  he  surely  will 


246  FEAGMENTS  OF  SCIENCE. 

permit  the  adherent  of  the  erosion  theory  to  make  a 
similar  assumption. 

The  influence  of  small  accidents  on  the  direction  of 
rivers  is  beautifully  illustrated  in  glacier  streams,  which 
are  made  to  cut  either  straight  or  sinuous  channels  by 
causes  apparently  of  the  most  trivial  character.  In 
his  interesting  paper  '  On  the  Lakes  of  Switzerland,' 
M.  Studer  also  refers  to  the  bend  of  the  Ehine  at 
Sargans  in  proof  that  the  river  must  there  follow  a 
pre-existing  fissure.  I  made  a  special  expedition  to  the 
place  in  1864 ;  and  though  it  was  plain  that  M.  Studer 
had  good  grounds  for  the  selection  of  this  spot,  I  was 
unable  to  arrive  at  his  conclusion  as  to  the  necessity  of 
a  fissure. 

Again,  in  the  interesting  volume  recently  published 
by  the  Swiss  Alpine  Club,  M.  Desor  informs  us  that 
the  Swiss  naturalists  who  met  last  year  at  Samaden 
visited  the  end  of  the  Morteratsch  glacier,  and  there 
convinced  themselves  that  a  glacier  had  no  tendency 
whatever  to  imbed  itself  in  the  soil.  I  scarcely  think 
that  the  question  of  glacier  erosion,  as  applied  either  to 
lakes  cr  valleys,  is  to  be  disposed  of  so  easily.  Let  me 
record  here  my  experience  of  the  Morteratsch  glacier. 
I  took  with  me  in  1864  a  theodolite  to  Pontresina, 
and  while  there  had  to  congratulate  myself  on  the 
aid  of  my  friend  Mr.  Hirst,  who  in  1857  did  such 
good  service  upon  the  Mer  de  Glace  and  its  tribu- 
taries. We  set  out  three  lines  across  the  Morteratsch 
glacier,  one  of  which  crossed  the  ice-stream  near  the 
well-known  hut  of  the  painter  Georgei,  while  the  two 
others  were  staked  out,  the  one  above  the  hut  and  the 
other  below  it.  Calling  the  highest  line  A,  the  line 
which  crossed  the  glacier  at  the  hut  B,  and  the  lowest 
line  C,  the  following  are  the  mean  liourly  motions  of 
the  three  lines,  deduced  from  observations  which  ex.- 


ALPINE  SCULPTURE.  247 

tended  over  several  days.  On  each  line  eleven  stakes 
were  fixed,  which  are  designated  by  the  figures  1,  2,  3, 
&c.  in  the  Tables. 

Morteratsch  Glacier ^  Line  A. 

No.  of  Stake.  Hourly  Motion* 

1 0-35  incli. 

2 0-49    „ 

3 0-53    „ 

4 0-54    „ 

6 0-56    „ 

6 0-54    „ 

7 0-52    „ 

8 0-49    „ 

9 0-40    „ 

10 0-29     „ 

11 0-20     „ 

As  in  all  other  measurements  of  this  kind,  the  re- 
tarding influence  of  the  sides  of  the  glacier  is  manifest : 
the  centre  moves  with  the  greatest  velocity. 

Morteratsch  Glacier ,  Line  B. 

No.  of  Stake  Hourly  Motion. 

1  .....        ,  0-05  inch. 

2 014    „ 

8 0-24    „ 

4 0-32    „ 

6 0-41     „ 

6 0-44    „ 

7  .  .        .        .        .        .  0-44    „ 

8  .....  .  0-45     „ 

9  ....  .  0-43    „ 

10 0-44     „ 

11 0-44     „ 

The  first  stake  of  this  line  was  quite  close  to  the 
edge  of  the  glacier,  and  the  ice  was  thin  at  the  place, 
hence  its  slow  motion.  Crevasses  prevented  us  from 
carrying  the  line  sufficiently  far  across  to  render  the 
retardation  of  the  further  side  of  the  glacier  fully 
evident. 

17 


248  FRAGMENTS  OF  SCIENCE. 

Morteratsch  Glacier ^  Line  C. 

Kow  of  Stake  Hourly  Motfco. 

1 0-05  inch. 

2 0-09    „ 

8 0-18    „ 

4 0-20    „ 

6 0-25    „ 

6 0-27    „ 

T  «        •        •        •        •        •  0*27    )) 

8  .        .        ,        ...        .  0-30    „ 

9 0-21    „ 

10 0-20    „ 

11 c  0-16    „ 

Comparing  the  three  lines  together,  it  will  be  ob- 
served that  the  velocity  diminishes  as  we  descend  the 
glacier.  In  100  hours  the  maximum  motion  of  the 
three  lines  respectively  is  as  follows  : 

Maximum  Motion  in  100  hours. 


Line  A   .        .        .        .        .56  inchea 

w    B 45      „ 

»    0 30      „ 


This  deportment  explains  an  appearance  which 
must  strike  every  observer  who  looks  upon  the  Mor- 
teratsch from  the  Piz  Languard,  or  from  the  new 
Bernina  Eoad.  A  medial  moraine  runs  along  the 
glacier,  commencing  as  a  narrow  streak,  but  towards 
the  end  the  moraine  extending  in  width,  until  finally 
it  quite  covers  the  terminal  portion  of  the  glacier. 
The  cause  of  this  is  revealed  by  the  foregoing  measure- 
ments, which  prove  that  a  stone  on  the  moraine  where 
it  is  crossed  by  the  line  A  approaches  a  second  stone 
on  the  moraine  where  it  is  crossed  by  the  line  C  with  a 
velocity  of  twenty-six  inches  per  one  hundred  hours. 
The  moraine  is  in  a  state  of  longitudinal  compression. 
Its  materials  are  more  and  more  squeezed  together, 
and  they  must  consequently  move  laterally  and  render 


ALPINE  SCULPTURE.  249 

the  moraine  at  the  terminal  portion  of  the  glacier 
wider  than  above. 

The  motion  of  the  Morteratsch  glacier,  then, 
diminishes  as  we  descend.  The  maximum  motion  of 
the  third  line  is  thirty  inches  in  one  hundred  hours,  or 
seven  inches  a  day — a  very  slow  motion ;  and  had  we 
run  a  line  nearer  to  the  end  of  the  glacier,  the  motion 
would  have  been  slower  still.  At  the  end  itself  it  is 
nearly  insensible.*  Now  I  submit  that  this  is  not  the 
place  to  seek  for  the  scooping  power  of  a  glacier.  The 
opinion  appears  to  be  prevalent  that  it  is  the  snout  of 
a  glacier  that  must  act  the  part  of  ploughshare  ;  and  it 
is  certainly  an  erroneous  opinion.  The  scooping  power 
will  exert  itself  most  where  the  weight  and  the  motion 
are  greatest.  A  glacier's  snout  often  rests  upon  matter 
which  has  been  scooped  from  the  glacier's  bed  higher 
up.  I  therefore  do  not  think  that  the  inspection  of 
what  the  end  of  a  glacier  does  or  does  not  accomplish 
can  decide  this  question. 

The  snout  of  a  glacier  is  potent  to  remove  anything 
against  which  it  can  fairly  abut ;  and  this  power,  not- 
withstanding the  slowness  of  the  motion,  manifests 
itself  at  the  end  of  the  Morteratsch  glacier.  A  hillock, 
bearing  pine-trees,  was  in  front  of  the  glacier  when 
Mr.  Hirst  and  myself  inspected  its  end ;  and  this 
hillock  is  being  bodily  removed  by  the  thrust  of  the 
ice.  Several  of  the  trees  are  overturned  ;  and  in  a  few 
years,  if  the  glacier  continues  its  reputed  advance,  the 
mound  will  certainly  be  ploughed  away. 

The  question  of  Alpine  conformation  stands,  I 
think,  thus :  We  have,  in  the  first  place,  great  valleys, 

*  The  snout  of  the  Aletsch  Glacier  has  a  diurnal  motion  of  less 
than  two  inches,  while  a  mile  or  so  above  the  snout  the  velocity  ia 
eighteen  inches.  The  spreading  out  of  the  moraine  is  here  very 
striking. 


250  FRAGMENTS  OF  SCIENCE. 

such  as  those  of  the  Rhine  and  the  Rhone,  which 
we  might  conveniently  call  valleys  of  the  first  order. 
The  mountains  which  flank  these  main  valleys  are 
also  cut  by  lateral  valleys  running  into  the  main 
ones,  and  which  may  be  called  valleys  of  the  second 
order.  When  these  latter  are  examined,  smaller 
valleys  are  found  running  into  them,  which  may  be 
called  valleys  of  the  third  order.  Smaller  ravines  and 
depressions,  again,  join  the  latter,  which  may  be  called 
valleys  of  the  fourth  order,  and  so  on  until  we  reach 
streaks  and  cuttings  so  minute  as  not  to  merit  the 
name  of  valleys  at  all.  At  the  bottom  of  every  valley 
we  have  a  stream,  diminishing  in  magnitude  as  the 
order  of  the  valley  ascends,  carving  the  earth  and 
carrying  its  materials  to  lower  levels.  We  find  that 
the  larger  valleys  have  been  filled  for  untold  ages  by 
glaciers  of  enormous  dimensions,  always  moving,  grind- 
ing down  and  tearing  away  the  rocks  over  which  they 
passed.  We  have,  moreover,  on  the  plains  at  the  feet 
of  the  mountains,  and  in  enormous  quantities,  the  very 
matter  derived  from  the  sculpture  of  the  mountains 
themselves. 

The  plains  of  Italy  and  Switzerland  are  cumbered 
by  the  debris  of  the  Alps.  The  lower,  wider,  and 
more  level  valleys  are  also  filled  to  unknown  depths 
with  the  materials  derived  from  the  higher  ones.  In 
tlie  vast  quantities  of  moraine-matter  which  cnmber 
many  even  of  the  higher  valleys  we  have  also  sugges- 
tions as  to  the  magnitude  of  the  erosion  which  has 
taken  place.  This  moraine-matter,  moreover,  can  only 
in  small  part  have  been  derived  from  the  falling  of 
rocks  upon  the  ancient  glacier ;  it  is  in  great  part 
derived  from  the  grinding  and  the  ploughing-out  of 
the  glacier  itself.  This  accounts  for  the  magnitude  of 
many  of  the    ancient   moraines,  which    date  from  a 


ALPINE  SCULPTURE.  251 

period  when  almost  all  tne  mountains  were  covered 
with  ice  and  snow,  and  when,  consequently,  the 
quantity  of  moraine-matter  derived  from  the  naked 
crests  cannot  have  been  considerable. 

The  erosion  theory  ascribes  the  formation  of  Alpine 
valleys  to  the  agencies  here  briefly  referred  to.  It 
invokes  nothing  but  true  causes.  Its  artificers  are 
still  there,  though,  it  may  be,  in  diminished  strength ; 
and  if  they  are  granted  sufficient  time,  it  is  demon- 
strable that  they  are  competent  to  produce  the  effects 
ascribed  to  them.  And  what  does  the  fracture  theory 
offer  in  comparison  ?  From  no  possible  application  of 
this  theory,  pure  and  simple,  can  we  obtain  the  slopes 
and  forms  of  the  mountains.  Erosion  must  in  the 
long  run  be  invoked,  and  its  power  therefore  conceded. 
The  fracture  theory  infers  from  the  disturbances  of  the 
Alps  the  existence  of  fissures ;  and  this  is  a  probable 
inference.  But  that  they  were  of  a  magnitude  sufficient 
to  produce  the  conformation  of  the  Alps,  and  that 
they  followed,  as  the  Alpine  valleys  do,  the  lines  of 
natural  drainage  of  the  country,  are  assumptions  which 
do  not  appear  to  me  to  be  justified  either  by  reason  or 
by  observation. 

There  is  a  grandeur  in  the  secular  integration  of 
small  effects  implied  by  the  theory  of  erosion  almost 
superior  to  that  involved  in  the  idea  of  a  cataclysm. 
Think  of  the  ages  which  must  have  been  consumed  in 
the  execution  of  this  colossal  sculpture.  The  question 
may,  of  course,  be  pushed  further.  Think  of  the  ages 
which  the  molten  earth  required  for  its  consolidation. 
But  these  vaster  epochs  lack  sublimity  through  our 
inability  to  grasp  them.  They  bewilder  us,  but  they 
fail  to  make  a  solemn  impression.  The  genesis  of  the 
mountains  comes  more  within  the  scope  of  the  intellect, 
and  the  majesty  of  the  operation  is  enhanced  by  our 


252  FEAGMENTS  OF  SCIENCE. 

partial  ability  to  conceive  it.  In  the  falling  of  a  rock 
from  a  mountain-head,  in  the  shoot  of  an  avalanche,  in 
the  plunge  of  a  cataract,  we  often  see  more  impressive 
illustrations  of  the  power  of  gravity  than  in  the  motions 
of  the  stars.  When  the  intellect  has  to  intervene,  and 
calculation  is  necessary  to  the  building  up  of  the  con- 
ception, the  expansion  of  the  feelings  ceases  to  be 
proportional  to  the  magnitude  of  the  phenomena. 


I  will  here  record  a  few  other  measurements  exe- 
cuted  on  the  Rosegg  glacier :  the  line  was  staked  out 
across  the  trunk  formed  by  the  junction  of  the  Rosegg 
proper  with  the  Tschierva  glacier,  a  short  distance 
below  the  rocky  promontory  called  Agaliogs. 

Eosegg  Glacier. 

No.  of  stake.  Hourly  Motion, 

1 001  inch. 

2 005     „ 

3 0-07     „ 

4 0-10    „ 

6 Oil     „ 

6 013    „ 

7  •.'..,        ,  014     „ 

8 018    „ 

9 0-24    „ 

10 0-23     „ 

11 0-24    „ 

This  is  an  extremely  slowly  moving  glacier;  the 
maximum  motion  hardly  amounts  to  seven  inches  a  day. 
Crevasses  prevented  us  from  continuing  the  line  quite 
across  the  glacier. 


X. 

REGENT  EXPERIMENTS   ON  FOG-SIGNALS.^ 

THE  care  of  its  sailors  is  one  of  the  first  duties  of  a 
maritime  people,  and  one  of  the  sailor's  greatest 
dangers  is  his  proximity  to  the  coast  at  night.  Hence 
the  idea  of  warning  him  of  such  proximity  by  beacon- 
fires  placed  sometimes  on  natural  eminences  and  some- 
times on  towers  built  expressly  for  the  purpose.  Close 
to  Dover  Castle,  for  example,  stands  an  ancient  Pharos 
of  this  description. 

As  our  marine  increased  greater  skill  was  invoked, 
and  lamps  reinforced  by  parabolic  reflectors  poured 
their  light  upon  the  sea.  Several  of  these  lamps  were 
sometimes  grouped  together  so  as  to  intensify  the  light, 
which  at  a  little  distance  appeared  as  if  it  emanated 
from  a  single  source.  This  '  catoptric '  form  of  appa- 
ratus is  still  to  some  extent  employed  in  our  lighthouse- 
service,  but  for  a  long  time  past  it  has  been  more  and 
more  displaced  by  the  great  lenses  devised  by  the  illus- 
trious Frenchman,  Fresnel. 

In  a  first-class  'dioptric'  apparatus  the  light 
emanates  from  a  lamp  with  several  concentric  wicks, 
the  flame  of  which,  being  kindled  by  a  very  active 
draught,  attains  to  great  intensity.  In  fixed  lights  the 
lenses  refract  the  rays  issuing  from  the  lamp  so  as  to 
cause  them  to  form  a  luminous  sheet  which  grazes  the 

*  A  discourse  delivered  in  the  Royal  Institution,  March  22, 1878. 


254  FRAGMENTS  OF  SCIENCE. 

sea-horizon.  In  revolving  lights  the  lenses  gather  up 
the  rays  into  distinct  beams,  resembling  the  spokes  of  a 
wheel,  which  sweep  over  the  sea  and  strike  the  eye  of 
the  mariner  in  succession. 

It  is  not  for  clear  weather  that  the  greatest  strength- 
ening of  the  light  is  intended,  for  here  it  is  not  needed. 
Nor  is  it  for  densely  foggy  weather,  for  here  it  is  in- 
effectual. But  it  is  for  the  intermediate  stages  of  hazy, 
snowy,  or  rainy  weather,  in  which  a  powerful  light  can 
assert  itself,  while  a  feeble  one  is  extinguished.  The 
usual  first-order  lamp  is  one  of  four  wicks,  but  Mr. 
Douglass,  the  able  and  indefatigable  engineer  of  the 
Trinity  House,  has  recently  raised  the  number  of  the 
wicks  to  six,  which  produce  a  very  noble  flame.  To  Mr. 
Wigham,  of  Dublin,  we  are  indebted  for  the  successful 
application  of  gas  to  lighthouse  illumination.  In  some 
lighthouses  his  power  varies  from  28  jets  to  108  jets, 
while  in  the  lighthouse  of  Galley  Head  three  burners  of 
the  largest  size  can  be  employed,  the  maximum  number 
of  jets  being  324.  These  larger  powers  are  invoked  only 
in  case  of  fog,  the  28-jet  burner  being  amply  sufficient 
for  clear  weather.  The  passage  from  the  small  burner 
to  the  large,  and  from  the  large  burner  to  the  small,  is 
made  with  ease,  rapidity,  and  certainty.  This  employ- 
ment of  gas  is  indigt^nous  to  Ireland,  and  the  Board 
of  Trade  has  exercised  a  wise  liberality  in  allowing 
every  facility  to  Mr.  Wigham  for  the  development  of 
his  invention. 

The  last  great  agent  employed  in  lighthouse  illu- 
mination is  electricity.  It  was  in  this  Institution, 
beginning  in  1831,  that  Faraday  proved  the  existence 
and  illustrated  the  laws  of  those  induced  currents  which 
in  our  day  have  received  such  astounding  development. 
In  relation  to  this  subject  Faraday's  words  have  a  pro- 
phetic ring.     *  I  have  rather,'  he  writes  in  1831, '  been 


RECENT  EXPERIMENTS  ON  FOG-SIGNALS.        255 

desirous  of  discovering  new  facts  and  new  relations  de- 
pendent on  magneto-electric  induction  than  of  exalting 
the  force  of  those  already  obtained,  being  assured  that 
the  latter  would  find  their  full  development  hereafter.' 
The  labours  of  Holmes,  of  the  Paris  Alliance  Company, 
of  Wilde,  and  of  Gramme,  constitute  a  brilliant  fulfil- 
ment of  this  prediction. 

But,  as  regards  the  augmentation  of  power,  the 
greatest  step  hitherto  made  was  independently  taken  a 
few  years  ago  by  Dr.  Werner  Siemens  and  Sir  Charles 
Wheatstone.  Through  the  application  of  their  dis- 
covery a  machine  endowed  with  an  infinitesimal  charge 
of  magnetism  may,  by  a  process  of  accumulation  at 
compound  interest,  be  caused  so  to  enrich  itself  mag- 
netically as  to  cast  by  its  performance  all  the  older 
machines  into  the  shade.  The  light  now  before  you  is 
that  of  a  small  machine  placed  downstairs,  and  worked 
there  by  a  minute  steam-engine.  It  is  a  light  of  about 
1000  caudles ;  and  for  it,  and  for  the  steam-engine  that 
works  it,  our  members  are  indebted  to  the  liberality  of 
Dr.  William  Siemens,  who  in  the  most  generous  manner 
has  presented  the  machine  to  this  Institution.  After 
an  exhaustive  trial  at  the  South  Foreland,  machines  on 
the  principle  of  Siemens,  but  of  far  greater  power  than 
this  one,  have  been  recently  chosen  by  the  Elder 
Brethren  of  the  Trinity  House  for  the  two  light-houses 
at  the  Lizard  Point. 

Our  most  intense  lights,  including  the  six-wick  lamp, 
the  Wigham  gas  light,  and  the  electric  light,  being 
intended  to  aid  the  mariner  in  heavy  weather,  may  be 
regarded,  in  a  certain  sense,  as  fog-signals.  But  fog, 
when  thick,  is  intractable  to  light.  The  sun  cannot 
penetrate  it,  much  less  any  terrestrial  source  of  illumi- 
nation. Hence  the  necessity  of  employing  sound-signals 
in  dense  fogs.     Bells,  gongs^  horns,  whistles,  guns,  and 


256  FRAGMENTS  OF  SCIENCE. 

syrens  have  been  used  for  this  purpose  ;  but  it  is  mainly, 
if  not  wholly,  with  explosive  signals  that  we  have  now 
to  deal.  The  gun  has  been  employed  with  useful  effect 
at  the  North  Stack,  near  Holyhead,  on  the  Kish  Bank 
near  Dublin,  at  Lundy  Island,  and  at  other  points  on 
our  coasts.  During  the  long,  laborious,  and  I  venture 
to  think  memorable  series  of  observations  conducted 
under  the  auspices  of  the  Elder  Brethren  of  the  Trinity 
House  at  the  South  Foreland  in  1872  and  1873,  it  was 
proved  that  a  short  5^-inch  howitzer,  firing  3  lbs.  of 
powder,  yielded  a  louder  report  than  a  long  18-pounder 
firing  the  same  charge.  Here  was  a  hint  to  be  acted  on 
by  the  Elder  Brethren.  The  effectiveness  of  the  sound 
depended  on  the  shape  of  the  gun,  and  as  it  could  not 
be  assumed  that  in  the  howitzer  we  had  hit  accidentally 
upon  the  best  possible  shape,  arrangements  were  made 
with  the  War  Office  for  the  construction  of  a  gun  spe- 
cially calculated  to  produce  the  loudest  sound  attainable 
from  the  combustion  of  3  lbs.  of  powder.  To  prevent 
the  unnecessary  landward  waste  of  the  sound,  the  gun 
was  furnished  with  a  parabolic  muzzle,  intended  to  pro- 
ject the  sound  over  the  sea,  where  it  was  most  needed. 
The  construction  of  this  gun  was  based  on  a  searching 
series  of  experiments  executed  at  Woolwich  with  small 
models,  provided  with  muzzles  of  various  kinds.  A 
drawing  of  the  gun  is  annexed  (p.  257).  It  was  con- 
structed on  the  principle  of  the  revolver,  its  various 
chambers  being  loaded  and  brought  in  rapid  succession 
into  the  firing  position.  The  performance  of  the  gun 
proved  the  correctness  of  the  prmciples  on  which  its 
construction  was  based. 

An  incidental  point  of  some  interest  was  decided  by 
the  earliest  Woolwich  experiments.  It  had  been  a 
widely  spread  opinion  among  artillerists,  that  a  bronze 
gun  produces  a  specially  loud  report.     I  doubted  from 


BECENT  EXPERIMENTS  ON  EOG-SIGNALS.        257 

the  outset  whether  this  would  help  us  ;  and  in  a  letter 
dated  22nd  April,  1874,  I  ventured  to  express  myself 
thus  : — '  The  report  of  a  gun,  as  affecting  an  observer 
close  at  hand,  is  made  up  of  two  factors — the  sound  due 
to  the  shock  of  the  air  by  the  violently  expanding  gas, 
and  the  sound  derived  from  the  vibrations  of  the  gun, 
which,  to  some  extent,  rings  like  a  bell.  This  latter,  I 
apprehend,  will  disappear   at    considerable  distances.' 


Fig.  6. 

Breech-loading  Fog-signal  Gun,  with  Bell  Mouth,'  proposed  by- 
Major  Maitland,  E.A.,  Assistant  Superintendent. 

The  result  of  subsequent  trial,  as  reported  by  General 
Campbell,  is,  '  that  the  sonorous  qualities  of  bronze  are 
greatly  superior  to  those  of  cast  iron  at  short  distances, 
but  that  the  advantage  lies  with  the  baser  metal  at 
long  ranges.'  ^ 

'  The  carriage  of  this  gun  has  been  modified  in  construction 
since  this  drawing  was  made. 

*  General  Campbell  assigns  a  true  cause  for  this  difference. 
The  ring  of  the  bronze  gun  represents  so  much  energy  withdrawn 


258  FRAGMENTS  OF  SCIENCE. 

Coincident  with  these  trials  of  guns  at  Woolwich, 
gun-cotton  was  thought  of  as  a  probably  effective  sound- 
producer.  From  the  first,  indeed,  theoretic  considera- 
tions caused  me  to  fix  my  attention  persistently  on  this 
substance  ;  for  the  remarkable  experiments  of  Mr.  Abel, 
whereby  its  rapidity  of  combustion  and  violently  ex- 
plosive energy  are  demonstrated,  seemed  to  single  it 
out  as  a  substance  eminently  calculated  to  fulfil  the 
conditions  necessary  to  the  production  of  an  intense 
wave  of  sound.  What  those  conditions  are  we  shall  now 
more  particularly  enquire,  calling  to  our  aid  a  brief  but 
very  remarkable  paper,  published  by  Professor  Stokes 
in  the  '  Philosophical  Magazine  '  for  1868. 

The  explosive  force  of  gunpowder  is  known  to  depend 
on  the  sudden  conversion  of  a  solid  body  into  an  in- 
tensely heated  gas.  Now  the  work  which  the  artillerist 
requires  the  expanding  gas  to  perform  is  the  displace- 
ment of  the  projectile,  besides  which  it  has  to  displace 
the  air  in  front  of  the  projectile,  which  is  backed  by 
the  whole  pressure  of  the  atmosphere.  Such,  however, 
is  not  the  work  that  we  want  our  gunpowder  to  per- 
form. We  wish  to  transmute  its  energy  not  into  the 
mere  mechanical  translation  of  either  shot  or  air,  but 
into  vibratory  motion.  We  want  pulses  to  be  formed 
which  shall  propagate  themselves  to  vast  distances 
through  the  atmosphere,  and  this  requires  a  certain 
choice  and  management  of  the  explosive  material. 

A  sound-wave  consists  essentially  of  two  parts — a 
condensation  and  a  rarefaction.  Now  air  is  a  very 
mobile  fluid,  and  if  the  shock  imparted  to  it  lack  due 
promptness,  the  wave  is  not  produced.  Consider  the 
case  of  a  common  clock  pendulum,  which  oscillates  to 

from  the  explosive  force  of  the  gunpowder.  Further  experiments 
would,  however,  be  needed  to  place  the  superiority  of  the  cast-iron 
gun  at  a  distance  beyond  question. 


EECENT   EXPERIxMENTS   ON   FOG-SING ALS,        259 

and  fro,  and  which  might  be  expected  to  generate  cor- 
responding pulses  in  the  air.  When,  for  example,  the 
bob  moves  to  the  right,  the  air  to  the  right  of  it  might 
be  supposed  to  be  condensed,  while  a  partial  vacuum 
might  be  supposed  to  follow  the  bob.  As  a  matter  of 
fact,  we  have  nothing  of  the  kind.  The  air  particles  in 
front  of  the  bob  retreat  so  rapidly,  and  tliose  behind  it 
close  so  rapidly  in,  that  no  sound-pulse  is  formed.  The 
mobility  of  hydrogen,  moreover,  being  far  greater  than 
that  of  air,  a  prompter  action  is  essential  to  the  forma- 
tion of  sonorous  waves  in  hydrogen  than  in  air.  It  is  to 
this  rapid  power  of  readjustment,  this  refusal,  so  to 
speak,  to  allow  its  atoms  to  be  crowded  together  or  to  be 
drawn  apart,  that  Professor  Stokes,  with  admirable 
penetration,  refers  the  damping  power,  first  described 
by  Sir  John  Leslie,  of  hydrogen  upon  sound. 

A  tuning-fork  which  executes  256  complete  vibra- 
tions in  a  second,  if  struck  gently  on  a  pad  and  held  in 
free  air,  emits  a  scarcely  audible  note.  It  behaves  to 
some  extent  like  the  pendulum  bob  just  referred  to. 
This  feebleness  is  due  to  the  prompt '  reciprocating  flow ' 
of  the  air  between  the  incipient  condensations  and  rare- 
factions, whereby  the  formation  of  sound-pulses  is  fore- 
stalled. Stokes,  however,  has  taught  us  that  this  flow 
may  be  intercepted  by  placing  the  edge  of  a  card  in 
close  proximity  to  one  of  the  corners  of  the  fork. 
An  immediate  augmentation  of  the  sound  of  the  fork  is 
the  consequence. 

The  more  rapid  the  shock  imparted  to  the  air,  the 
greater  is  the  fractional  part  of  the  energy  of  the  shock 
converted  into  wave  motion.  And  as  different  kinds  of 
gunpowder  vary  considerably  in  their-  rapidity  of  com- 
bustion, it  may  be  expected  thai  they  will  also  vary  as 
producers  of  sound.  This  theoretic  inference  is  com- 
pletely verified  by  experiment.     In  a  series  of  prelimi- 


L.G. 

R.  L.  a. 

Fig.  7. 

260  FKAGMENTS   OF  SCIENCE. 

nary  trials  conducted  at  Woolwich  on  the  4th  of  June, 
1875,  the  sound-producing  powers  of  four  different 
kinds  of  powder  were  determined.  In  the  order  of  the 
size  of  their  grains  they  bear  the  names  respectively  of 
Fine-grain  (F.  Gr.),  Large-grain  (L.  Gr.),  Eifle  Large- 
grain  (R.  L.  Gr.),  and  Pebble- powder  (P.)  (See  annexed 
figures.)     The  charge  in  each  case  amounted  to  4^  lbs. ; 


P.O.  L.G.  R.L.G.  P. 


four  24-lb.  howitzers  being  employed  to  fire  the  respec- 
tive charges.  There  were  eleven  observers,  all  of  whom, 
without  a  single  dissentient,  pronounced  the  sound  of 
the  fine-grain  powder  loudest  of  all.  In  the  opinion  of 
seven  of  the  eleven  the  large-grain  powder  came  next ; 
seven  also  of  the  eleven  placed  the  rifle  large-grain  third 
on  the  list ;  while  they  were  again  unanimous  in  pro- 
nouncing the  pebble-powder  the  worst  sound-producer 
These  differences  are  entirely  due  to  differences  in  the 
rapidity  of  combustion.  All  who  have  witnessed  the 
performance  of  the  80-ton  gun  must  have  been  sur- 
prised at  the  mildness  of  its  thunder.  To  avoid  the 
strain  resulting  from  quick  combustion,  the  powder 
employed  is  composed  of  lumps  far  larger  than  those 
of  the  pebble-powder  above  referred  to.  In  the  long 
tube  of  the  gun  these  lumps  of  solid  matter  gradually 
resolve  themselves  into  gas,  which  on  issuing  from  the 
muzzle  imparts  a  kind  of  push  to  the  air,  instead  of 


RECENT  EXPERIMENTS   ON  FOG-SIGNALS.        261 

the  sharp  shock  necessary  to  form  the  condensation  of 
an  intensely  sonorous  wave. 

These  are  some  of  the  physical  reasons  why  gun- 
cotton  might  be  regarded  as  a  promising  fog-signal. 
Firing  it  as  we  have  been  taught  to  do  by  Mr.  Abel, 
its  explosion  is  more  rapid  than  that  of  gunpowder. 
In  its  case  the  air  particles,  alert  as  they  are,  will  not, 
it  might  be  presumed,  be  able  to  slip  from  condensation 
to  rarefaction  with  a  rapidity  sufficient  to  forestall  the 
formation  of  the  wave.  On  a  ^priori  grounds  then,  we 
are  entitled  to  infer  the  effectiveness  of  gun-cotton, 
while  in  a  great  number  of  comparative  experiments, 
stretching  from  1874  to  the  present  time,  this  inference 
has  been  verified  in  the  most  conclusive  manner. 

As  regards  explosive  material,  and  zealous  and 
accomplished  help  in  the  use  of  it,  the  resources  of 
Woolwich  Arsenal  have  been  freely  placed  at  the  dis- 
posal of  the  Elder  Brethren.  General  Campbell,  Grene- 
ral  Younghusband,  Colonel  Fraser,  Colonel  Maitland, 
and  other  officers,  have  taken  an  active  personal  part  in 
the  investigation,  and  in  most  cases  have  incurred  the 
labour  of  reducing  and  reporting  on  the  observations. 
Gruns  of  various  forms  and  sizes  have  been  invoked  for 
gunpowder,  while  gun-cotton  has  been  fired  in  free  air 
and  in  the  foci  of  parabolic  reflectors. 

On  the  22nd  of  February,  1875,  a  number  of  small 
guns,  cast  specially  for  the  purpose — some  with  plain, 
some  with  conical,  and  some  with  parabolic  muzzles — 
firing  4  oz.  of  fine  grain  powder,  were  pitted  against 
4  oz.  of  gun-cotton  detonated  both  in  the  open,  and  in 
{ he  focus  of  a  parabolic  reflector,^  The  sound  produced 
by  the  gun-cotton,  reinforced  by  the  reflector,  was 
unanimously  pronounced       adest  of  all.     With  equal 

'  For  charges  of  this  weig  at  the  reflector  is  of  moderate  size, 
and  may  be  employed  without  fear  of  fracture. 


262 


FKAGMENTS  OF  SCIENCE. 


unaDimity,  the  gun-cotton  detonated  in  free  air  waa 
placed  second  in  intensity.  Though  the  same  charge 
was  used  throughout,  the  guns  diifered  notably  among 
themselves,  but  none  of  them  came  up  to  the  gun-cotton, 
either  with  or  without  the  reflector.  A  second  series, 
observed  from  a  different  distance  on  the  same  day, 
confirmed  to  the  letter  the  foregoing  result. 

As  a  practical  point,  however,  the  comparative  cost 
of  gun-cotton  and  gunpowder  has  to  be  taken  into 
account,  though  considerations  of  cost  ought  not  to  be 
stretched  too  far  in  cases  involving  the  safety  of  human 
life.  In  the  earlier  experiments,  where  quantities  of 
equal  price  were  pitted  against  each  other,  the  results 
were  somewhat  fluctuating.  Indeed,  the  perfect  mani- 
pulation of  the  gun-cotton  required  some  preliminary 
discipline — promptness,  certainty,  and  effectiveness  of 
tiring,  augmenting  as  experience  increased.  As  1  lb.  of 
gun-cotton  costs  as  much  as  3  lbs.  of  gunpowder,  these 
quantities  were  compared  together  on  the  22nd  of  Feb- 
ruary. The  guns  employed  to  discharge  the  gunpowder 
were  a  1 2-lb.  brass  howitzer,  a  24-lb.  cast-iron  howitzer, 
and  the  long  18-pounder  employed  at  the  South  Fore- 
land. The  result  was,  that  the  24-lb.  howitzer,  firing 
3  lbs.  of  gunpowder,  had  a  slight  advantage  over  1  lb. 
of  gun-cotton  detonated  in  the  open ;  while  the  12-lb. 
howitzer  and  the  18-pounder  were  both  beaten  by  the 
gun-cotton.  On  the  2nd  of  May,  on  the  other  hand,  the 
gun-cotton  is  reported  as  having  been  beaten  by  all 
the  guns. 

Meanwhile,  the  parabolic-muzzle  gun,  expressly 
intended  for  fog-signalling,  was  pushed  rapidly  forward, 
and  on  March  22  and  23,  1876,  its  power  was  tested  at 
Shoeburyness.  Pitted  against  it  were  a  16-pounder,  a 
5i-inch  howitzer,  1^  lb.  of  gun-cotton  detonated  in  the 
focus  of  a  reflector  (see  annexed  figure),  and  1^  lb.  of 


RECENT  EXPERIMENTS  ON  FOO-SIGNALS, 


263 


gun-cotton  detonated  in  free  air.  On  this  occasion 
nineteen  different  series  of  experiments  were  made,  when 
the  new  experimental  gun,  firing  a  3-lb.  charge,  demon- 
strated its  superiority  over  all  guns  previously  employed 
to  fire  the  same  charge.     As  regards  the  comparative 


l^/V 


Fig.    8. 

Gun-cotton  Slab  (1^  lb.)  Detonated  in  the  Focus  of  a  Cast-iron 

Reflector. 

merits  of  the  gun-cotton  fired  in  the  open,  and  the  gun- 
powder fired  from  the  new  gun,  the  mean  values  of  their 
sounds  were  the  same.  Fired  in  the  focus  of  the  re- 
flector, the  gun-cotton  clearly  dominated  over  all  the 
other  sound-producers.' 

The  whole  of  the  observations  here  referred  to  were 
embraced  by  an  angle  of  about  70°,  of  which  50°  lay  on 
'  The  reflector  was  fractured  by  the  explosion,  but  it  did  good 
service  afterwards. 

18 


264  FKAGMENTS  OF  SCIENCE. 

the  one  side  and  20"*  on  the  other  side  of  the  line  of  fire. 
The  shots  were  heard  by  eleven  observers  on  board  the 
'  Galatea,'  which  took  up  positions  varying  from  2  miles 
to  13J  miles  from  the  firing-point.  In  all  these  obser- 
vations, the  reinforcing  action  of  the  reflector,  and  of 
the  parabolic  muzzle  of  the  gun,  came  into  play.  But 
the  reinforcement  of  the  sound  in  one  direction  implies 
its  withdrawal  from  some  other  direction,  and  accord- 
ingly it  was  found  that  at  a  distance  of  5 J  miles  from 
the  firing-point,  and  on  a  line  including  nearly  an  angle 
of  90°  with  the  line  of  fire,  the  gun-cotton  in  the  open 
beat  the  new  gun;  while  behind  the  station,  at  distances 
of  8 J  miles  and  13J  miles  respectively,  the  gun-cotton 
in  the  open  beat  both  the  gun  and  the  gun-cotton  in  the 
reflector.  This  result  is  rendered  more  important  by 
the  fact  that  the  sound  reached  the  Mucking  Light,  a 
distance  of  13  J  miles,  against  a  light  wind  which  was 
blowing  at  the  time. 

Most,  if  not  all,  of  our  ordinary  sound-producers 
send  forth  waves  which  are  not  of  uniform  intensity 
throughout.  A  trumpet  is  loudest  in  the  direction  of 
its  axis.  The  same  is  true  of  a  gun.  A  bell,  with  its 
mouth  pointed  upwards  or  downwards,  sends  forth  waves 
which  are  far  denser  in  the  horizontal  plane  passing 
through  the  bell  than  at  an  angular  distance  of  90°  from 
that  plane.  The  oldest  bellhangers  must  have  been 
aware  of  the  fact  that  the  sides  of  the  bell,  and  not  its 
mouth,  emitted  the  strongest  sound,  their  practice  being 
probably  determined  by  this  knowledge.  Our  slabs  of 
gun-cotton  albo  emit  waves  of  different  densities  in  differ- 
ent parts.  It  has  occurred  in  the  experiments  atShoebury- 
ness  thatwhen  the  broad  side  of  a  slab  was  turned  towards 
the  suspending  wire  of  a  second  slab  six  feet  distant,  the 
wire  was  cut  by  the  explosion,  while  when  the  edge  of 
the  slab  was  turned  to  the  wire  this  never  occurred. 


RECENT  EXPEKIMENTS  ON  FOG-SIGNALS.         265 

To  the  circumstance  that  the  broadsides  of  the  slabs 
faced  the  sea  is  probably  to  be  ascribed  the  remarkable 
fact  observed  on  March  23,  that  in  two  directions,  not 
tar  removed  from  the  line  of  fire,  the  gun-cotton 
detonated  in  the  open  had  a  slight  advantage  over  the 
new  gun. 

Theoretic  considerations  rendered  .it  probable  that 
the  shape  and  size  of  the  exploding  mass  would  affect 
the  constitution  of  the  wave  of  sound.  I  did  not  think 
large  rectangular  slabs  the  most  favourable  shape,  and 
accordingly  proposed  cutting  a  large  slab  into  fragments 
of  different  sizes,  and  pitting  them  against  each  other. 
The  differences  between  the  sounds  were  by  no  means 
so  great  as  the  differences  in  the  quantities  of  explosive 
material  might  lead  one  to  expect.  The  mean  values 
of  eighteen  se'ries  of  observations  made  on  board  the 
*Gralatea,'  at  distances  varying  from  1  j  mile  to  4*8  miles, 
were  as  follows: — 

Weights      .        ,        4  oz.        6  oz.        9  oz.        12  oz. 
Value  of  scund    .        3-12         3-34         4-0  403 

These  charges  were  cut  from  a  slab  of  dry  gun-cotton 
about  1|  inch  thick:  they  were  squares  and  rectangles 
of  the  following  dimensions: — 4  oz.,  2  inches  by  2 
inches;  6  oz.,  2  inches  by  3  inches;  9  oz.,  3  inches  by 
3  inches;  12  oz.,  2  inches  by  6  inches. 

The  numbers  under  the  respective  weights  express 
the  recorded  value  of  the  sounds.  They  must  be  simply 
taken  as  a  ready  means  of  expressing  the  approximate 
relative  intensity  of  tlie  sounds  as  estimated  by  the  ear. 
When  we  find  a  9-oz.  charge  marked  4,  and  a  12-oz. 
charge  marked  4*03,  the  two  sounds  may  be  regarded 
as  practically  equal  in  intensity,  thus  proving  that  an 
addition  of  30  per  cent,  in  the  larger  charges  produces 
no  sensible  difference  in  the  sound.  Were  tlie  sounds 
estimated  by  some  physical  means,  instead  of  by  the  ear, 


266  FEAGMENTS  OF  SCIENCE. 

the  values  of  the  sounds  at  the  distances  recorded  would 
not,  in  my  opinion,  show  a  greater  advance  with  the  in- 
crease of  material  than  that  indicated  by  the  foregoing 
numbers.  Subsequent  experiments  rendered  still  more 
certain  the  effectiveness,  as  well  as  the  economy,  of  the 
smaller  charges  of  gun-cotton. 

It  is  an  obvious  corollary  from  the  foregoing  experi- 
ments that  on  our  'nesses'  and  promontories,  where  the 
land  is  clasped  on  both  sides  for  a  considerable  distance 
by  the  sea — where,  therefore,  the  sound  has  to  propagate 
itself  rearward  as  well  as  forward — the  use  of  the  para- 
bolic gun,  or  of  the  parabolic  reflector,  might  be  a 
disadvantage  rather  than  an  advantage.  Here  gun- 
cotton,  exploded  in  the  open,  forms  the  most  appropriate 
source  of  sound.  This  remark  is  especially  applicable 
to  such  lightships  as  are  intended  to  spread  the  sound 
all  round  them  as  from  central  foci.  As  a  signal  in 
rock  lighthouses,  where  neither  syren,  steam-whistle, 
nor  gun  could  be  mounted ;  and  as  a  handy  fleet-signal, 
dispensing  with  the  lumber  of  special  signal-guns,  the 
gun-cotton  will  prove  invaluable.  But  in  most  of  these 
cases  we  have  the  drawback  that  local  damage  may  be 
done  by  the  explosion.  The  lantern  of  the  rock  light>- 
house  might  suffer  from  concussion  near  at  hand,  and 
though  mechanical  arrangements  might  be  devised, 
both  in  the  case  of  the  lighthouse  and  of  the  ship's  deck, 
to  place  the  firing-point  of  the  gun-cotton  at  a  safe 
distance,  no  such  arrangement  could  compete,  as  regards 
simplicity  and  effectiveness,  with  the  expedient  of  a 
gun-cotton  rocket.  Had  such  a  means  of  signalling 
existed  at  the  Bishop's  Eock  lighthouse,  the  ill-fated 
'  Schiller'  might  have  been  warned  of  her  approach  to 
danger  ten,  or  it  may  be  twenty,  miles  before  she  reached 
the  rock  which  wrecked  her.  Had  the  fleet  possessed 
such  a  signal,  instead  of  the  ubiquitous  but  ineffectuaJ 


RECENT  EXPERIMENTS  ON  FOG-SIGNALS.         267 

whistle,  the  'Iron  Duke'  and  'Vanguard'  need  never 
have  come  into  collision. 

It  was  the  necessity  of  providing  a  suitable  signal 
for  rock  lighthouses,  and  of  clearing  obstacles  which 
cast  an  acoustic  shadow,  that  suggested  the  idea  of  the 
gun-cotton  rocket  to  Sir  Kichard  Collinson,  Deputy 
Master  of  the  Trinity  House.  His  idea  was  to  place  a 
disk  or  short  cylinder  of  gun-cotton  in  the  head  of  a 
rocket,  the  ascensional  force  of  which  should  be  employed 
to  carry  the  disk  to  an  elevation  of  1000  feet  or  there- 
abouts, where  by  the  ignition  of  a  fuse  associated  with 
a  detonator,  the  gun-cotton  should  be  fired,  sending  its 
sound  in  all  directions  vertically  and  obliquely  down 
upon  earth  and  sea.  The  first  attempt  to  realise  this 
idea  was  made  on  July  18,  1876,  at  the  firework  manu- 
factory of  the  Messrs.  Brock,  at  Nunhead.  Eight 
rockets  were  then  fired,  four  being  charged  with  5  oz. 
and  four  with  7^  oz.  of  gun-cotton.  They  ascended  to 
a  great  height,  and  exploded  with  a  very  loud  report  in 
the  air.  On  July  27,  the  rockets  were  tried  at  Shoe- 
buryness.  The  most  noteworthy  result  on  this  occasion 
was  the  hearing  of  the  sounds  at  the  Mouse  Lighthouse, 
8^  miles  E.  by  S.,  and  at  the  Chapman  Lighthouse,  8J 
miles  W.  by  N. ;  that  is  to  say,  at  opposite  sides  of  the 
firing-point.  It  is  worthy  of  remark  that,  in  the  case 
of  the  Chapman  Lighthouse,  land  and  trees  intervened 
between  the  firing-point  and  the  place  of  observation 
'  This,'  as  General  Younghusband  justly  remarked  at 
the  time,  *  may  prove  to  be  a  valuable  consideration  if 
it  should  be  found  necessary  to  place  a  signal  station  in 
a  position  whence  the  sea  could  not  be  freely  observed.' 
Indeed,  the  clearing  of  such  obstacles  was  one  of  the 
objects  which  the  inventor  of  the  rocket  had  in  view. 

With  reference  to  the  action  of  the  wind,  it  was 
thought  desirable  to  compare  the  range  of  explosions 


268  FEAGMENTS   OF  SCIENCE. 

produced  near  the  surface  of  the  earth  with  othen 
produced  at  the  elevation  attainable  by  the  gun-cotton 
rockets.  Wind  and  weather,  however,  are  not  at  our 
command;  and  hence  one  of  the  objects  of  a  series  of 
experiments  conducted  on  December  13,  1876,  was  not 
fulfilled.  It  is  worthy,  however,  of  note  that  on  this 
day,  with  smooth  water  and  a  calm  atmosphere,  the 
rockets  were  distinctly  heard  at  a  distance  of  11*2  miles 
from  the  firing-point.  The  quantity  of  gun-cotton 
employed  was  7^  oz.  On  Thursday,  iVTarch  8,  1877, 
these  comparative  experiments  of  firing  at  high  and  low 
elevations  were  pushed  still  further.  The  gun-cotton 
near  the  ground  consisted  of  J-lb.  disks,  suspended  from 
a  horizontal  iron  bar  about  4^  feet  above  the  ground. 
The  rockets  carried  the  same  quantity  of  gun-cotton  in 
their  heads,  and  the  height  to  which  they  attained,  as 
determined  by  a  theodolite,  was  from  800  to  900  feet. 
The  day  was  cold,  with  occasional  squalls  of  snow  and 
hail,  the  direction  of  the  sound  being  at  right  angles  to 
that  of  the  wind.  Five  series  of  observations  were  made 
on  board  the  '  Vestal,'  at  distances  varying  from  3  to  6 
miles.  The  mean  value  of  the  explosions  in  the  air 
exceeded  that  of  the  explosions  near  the  ground  by 
a  small  but  sensible  quantity.  At  Windmill  Hill, 
Gravesend,  however,  which  was  nearly  to  leeward,  and 
5^  miles  from  the  firing-point,  in  nineteen  cases  out  of 
twenty-four  the  disk  fired  near  the  ground  was  loudest; 
while  in  the  remaining  five  the  rocket  had  the  ad- 
vantage. 

Towards  the  close  of  the  day  the  atmosphere  became 
very  serene.  A  few  distant  cumuli  sailed  near  the 
horizon,  but  the  zenith  and  a  vast  angular  space  all 
round  it  were  absolutely  free  from  cloud  From  the 
deck  of  the  '  Galatea'  a  rocket  was  discharged,  which 
reached  a  great  elevation,  and  exploded  with  a  loud 


EECENT  EXPEKIMENTS  ON  FOG-SIGNALS.  269 

report.  Following  this  solid  nucleus  of  sound  was  a 
continuous  train  of  echoes,  which  retreated  to  a  con- 
tinually greater  distance,  dying  gradually  off  into  silence 
after  seven  seconds'  duration.  These  echoes  were  of 
the  same  character  as  those  so  frequently  noticed  at 
the  South  Foreland  in  1872-73,  and  called  by  me  'aerial 
echoes.' 

On  the  23rd  of  March  the  experiments  were  re- 
sumed, the  most  noteworthy  results  of  that  day's  obser- 
vations being  that  the  sounds  were  heard  at  Tillingham, 
10  miles  to  the  N.E. ;  at  West  Mersea,  15|  miles  to  the 
N.E.  by  E. ;  at  Brightlingsea,  17^  miles  to  the  N.E. ; 
and  at  Clacton  Wash,  20^  miles  to  the  N.E.  by  ^  E. 
The  wind  was  blowing  at  the  time  from  the  S.E.  Some 
of  these  sounds  were  produced  by  rockets,  some  by  a  24- 
Ib.  howitzer,  and  some  by  an  8-inch  Maroon. 

In  December,  1876,  Mr.  Grardiner,  the  managing 
director  of  the  Cotton-powder  Company,  had  proposed 
a  trial  of  this  material  against  the  gun-cotton.  The 
density  of  the  cotton  he  urged  was  only  1'03,  while  that 
of  the  powder  was  1  •70.  A  greater  quantity  of  explo- 
sive material  being  thus  compressed  into  the  same 
volume,  Mr.  Gardiner  thought  that  a  greater  sonorous 
effect  must  be  produced  by  the  powder.  At  the  in- 
stance of  Mr.  Mackie,  who  had  previously  gone  very 
thoroughly  into  the  subject,  a  Committee  of  tlie  Elder 
Brethren  visited  the  cotton-powder  manufactory,  on  the 
banks  of  the  Swale,  near  Favershara,  on  the  1 6th  of 
June,  1877.  The  weights  of  cotton-powder  employed 
were  2  oz.,  8  oz.,  1  lb.,  and  2  lbs.,  in  the  form  of  rockets 
and  of  signals  fired  a  few  feet  above  the  ground.  The 
experiments  throughout  were  arranged  and  conducted 
by  Mr.  Mackie.  Our  desire  on  this  occasion  was  to  get 
as  near  to  windward   as   possible,  but  the   Swale  and 


270  FRAGMENTS  OF  SCIENCE. 

other  obstacles  limited  our  distance  to  1 J  mile.  We 
stood  here  E.S.E.  from  the  firing-point  while  the  wind 
blew  fresh  from  the  N.E. 

The  cotton-powder  yielded  a  very  effective  report. 
The  rockets  in  general  had  a  slight  advantage  over  the 
same  quantities  of  material  fired  near  the  ground.  The 
loudness  of  the  sound  was  by  no  means  proportional  to 
the  quantity  of  the  material  exploded,  8  oz.  yielding 
very  nearly  as  loud  a  report  as  1  lb.  The  '  aerial 
echoes,'  which  invariably  followed  the  explosion  of  the 
rockets,  were  loud  and  long- continued. 

On  the  17th  of  October,  1877,  another  series  of  ex- 
periments with  howitzers  and  rockets  was  carried  out  at 
Shoeburyness.  The  charge  of  the  howitzer  was  3  lbs. 
of  L.  G.  powder.  The  charges  of  the  rockets  were 
12  oz.,  8  oz.,  4  oz.,  and  2  oz.  of  gun-cotton  respectively. 
The  gun  and  the  four  rockets  constituted  a  series,  and 
eight  series  were  fired  during  the  afternoon  of  the  17th. 
The  observations  were  made  from  the  '  Vestal '  and  the 
*  Galatea,'  positions  being  successively  assumed  which 
permitted  the  sound  to  reach  the  observers  with  the 
wind,  against  the  wind,  and  across  the  wind.  The  dis- 
tance of  the  '  Galatea'  varied  from  3  to  7  miles,  that  of 
the  '  Vestal,'  which  was  more  restricted  in  her  move- 
ments, being  2  to  3  miles.  Briefly  summed  up,  the 
result  is  that  the  liowitzer,  firing  a  3-lb.  charge,  which 
it  will  be  remembered  was  our  best  gun  at  the  South 
Foreland,  was  beaten  by  the  1 2-oz.  rocket,  by  the  8-oz. 
rocket,  and  by  the  4-oz.  rocket.  The  2-oz.  rocket 
alone  fell  behind  the  howitzer. 

It  is  worth  while  recording  the  distances  at  which 
some  of  the  sounds  were  heard  on  the  day  now  re- 
ferred to : — 


BECENT  EXPERIMENTS  ON  FOG-SIGNALS.  271 

24  out  of  40  sounds  heard. 


I.  I^igh       .        . 

6^  miles  W.N.W. 

24  > 

S.  Girdler  Light- 

vessel  . 

12      „ 

S.B.  by  B. 

5 

3.  Reculvers 

m  » 

S.E.  by  S. 

18 

4.  St.  Nicholas    . 

20      „ 

S.E.    . 

3 

5.  Bpple  Bay 

22      „ 

S.E.  by  E. 

19 

6.  Westgate 

23      „ 

S.E.  by  B. 

9 

7.  Kingsgate 

25      „ 

S.E.  by  B. 

8 

The  day  was  cloudy,  with  occasional  showers  of 
drizzling  rain  ;  the  wind  about  N.W.  by  N.  all  day ;  at 
times  squally,  rising  to  a  force  of  6  or  7  and  sometimes 
dropping  to  a  force  of  2  or  3.  The  station  at  Leigh 
excepted,  all  these  places  were  to  leeward  of  Shoebury- 
ness.  At  four  other  stations  to  leeward,  varying  in 
distance  from  15^  to  24^  miles,  nothing  was  heard, 
while  at  eleven  stations  to  windward,  varying  from  8  to 
26  miles,  the  sounds  were  also  inaudible.  It  was  found, 
indeed,  that  the  sounds  proceeding  directly  against  the 
wind  did  not  penetrate  much  beyond  3  miles. 

On  the  following  day,  viz.  the  18th  October,  we 
proceeded  to  Dungeness  with  the  view  of  making  a 
series  of  strict  comparative  experiments  with  gun-cotton 
and  cotton-powder.  Eockets  containing  8  oz.,  4  oz., 
and  2  oz.  of  gun-cotton  had  been  prepared  at  the  Eoyal 
Arsenal ;  while  others,  containing  similar  quantities  of 
cotton-powder,  had  been  supplied  by  the  Cotton-powder 
Company  at  Faversham.  With  these  were  compared 
the  ordinary  18-pounder  gun,  which  happened  to  be 
mounted  at  Dungeness,  firing  the  usual  charge  of  3  lbs. 
of  powder,  and  a  syren. 

From  these  experiments  it  appeared  that  the  gun- 
cotton  and  cotton-powder  were  practically  equal  as 
producers  of  sound. 

The  effectiveness  of  small  charges  was  illustrated  in 
a  very  striking  manner,  only  a  single  unit  separating 
the  numerical  value  of  the  8-oz.  rocket  from  that  of  the 


272  FRAaMENTS  OF  SCIENCE. 

2-oz.  rocket.  The  former  was  recorded  as  6*9  and  the 
latter  as  5'9,  the  value  of  the  4-oz.  rocket  being  inter- 
mediate between  them.  These  results  were  recorded  by 
a  number  of  very  practised  observers  on  board  the 
*G-alatea.'  They  were  completely  borne  out  by  the 
observations  of  the  Coastguard,  who  marked  the  value 
of  the  8-oz  rocket  6'1,  and  that  of  the  2-oz.  rocket  5'2, 
The  18-pounder  gun  fell  far  behind  all  the  rockets,  a 
result,  possibly,  to  be  in  part  ascribed  to  the  imperfec- 
tion of  the  powder.  The  performance  of  the  syren  was, 
on  the  whole,  less  satisfactory  than  that  of  the  rocket. 
The  instrument  was  worked,  not  by  steam  of  70  lbs. 
pressure,  as  at  the  South  Foreland,  but  by  compressed 
air,  beginning  with  40  lbs.  and  ending  with  30  lbs. 
pressure.  The  trumpet  was  pointed  to  windward,  and 
in  the  axis  of  the  instrument  the  sound  was  about  as 
effective  as  that  of  the  8-oz.  rocket.  But  in  a  direction 
at  right  angles  to  the  axis,  and  still  more  in  the  rear  of 
this  direction,  the  syren  fell  very  sensibly  behind  even 
the  2-oz.  rocket. 

These  are  the  principal  comparative  trials  made  be- 
tween the  gun-cotton  rocket  and  other  fog-signals  ;  but 
they  are  not  the  only  ones.  On  the  2nd  of  August, 
1877,  for  example,  experiments  were  made  at  Lundy 
Island  with  the  following  results.  At  2  miles  distant 
from  the  firing-point,  with  land  intervening,  the  18- 
pounder,  firing  a  3-lb.  charge,  was  quite  unheard. 
Both  the  4-oz.  rocket  and  the  8-oz.  rocket,  however, 
reached  an  elevation  which  commanded  the  acoustic 
shadow,  and  yielded  loud  reports.  When  both  were  in 
view  the  rockets  were  still  superior  to  the  gun.  On 
the  6th  of  August,  at  St.  Ann's,  the  4-oz.  and  8-oz. 
rockets  proved  superior  to  the  syren.  On  the  Shambles 
Light- vessel,  when  a  pressure  of  13  lbs.  was  employed 
to  sound  the  syren,  the  rockets  proved  greatly  superior 


RECENT  EXPERIMENTS  ON  FOG-SIGNALS. 


273 


to  that  instrument.  Proceeding  along  the  sea  margin 
at  Flamboro'  Head,  Mr.  Edwards  states  that  at  a  dis- 
tance of  1;^  mile,  with  the  18-pounder  previously  used 
as  a  fog-signal  hidden  behind  the  cliffs,  its  report  was 
quite  unheard,  while  the  4-oz.  rocket,  rising  to  an  eleva- 
tion which  brought  it  clearly  into  view,  yielded  a  power- 
ful sound  in  the  face  of  an  opposing  wind. 

On  the  evening  of  February  9th,  1877,  a  remarkable 
series  of  experiments  were  made  by  Mr.  Prentice  at 
Stowmarket  with  the  gun-cotton  rocket.  From  the 
report  with  which  he  has  kindly  furnished  me  I  extract 
the  following  particulars.  The  first  column  in  the 
annexed  statement  contains  the  name  of  the  place  of 
observation,  the  second  its  distance  from  the  firing-point, 
and  the  third  the  result  observed  : — 

Stoke  Hill,  Ipswich        •  10  miles  Rockets  clearly  seen  and  sounda 

distinctly  heard  63  seconds 
after  the  flash. 

Melton   •        •        •  15      M      Signals        distinctly        heard. 

Thought  at  first  that  sounds 
were  reverberated  from   the 


Framlingham , 


Stratford.     St.  Andrews  .  19 


Tuddenham.     St.  Martin  10 


CJhrist  Church  Park. 


Nettlestead  Hall 


Bildestone 


Nacton 


18     ,f      Signals  very  distinctly  heard, 
both  in  the  open  air  and  in  a 
closed  room.     Wind  in  favour 
of  sound. 
Reports  loud ;  startled  pheasants 

in  a  cover  close  by. 
Reports  very  loud  ;  rolled  away 
like  thunder. 
11      „       Report    arrived  a  little    more 
than  a  minute  after  flash. 
6      „       Distinct  in  every  part  of  ob- 
server's house.     Very  loud  in 
the  open  air. 
6  •    „       Explosion    very      loud,     wind 
against  sound. 
14      „      Reports    quite    distinct  —  mis- 
taken by  inhabitants  for  clapa 
of  thunder. 


274  FEAGMENTS  OF  SCIENCE. 

Aldboio*  •        •        •        .25  miles  Bockets  seen    through  a    very 

hazy  atmosphere ;  a  rumbling 
detonation  heard. 

Capel  Milla  •  .  .  11  „  Reports  heard  within  and  with- 
out the  observer's  house. 
Wind  opposed  to  sound. 

Lawford  .        •        •        •  15^   ^      Reports  distinct :  attributed  to 

distant  thunder. 

In  the  great  majority  of  these  cases,  the  direction  of 
the  sound  enclosed  a  large  angle  with  the  direction 
of  the  wind.  In  some  cases,  indeed,  the  two  directions 
were  at  right  angles  to  each  other.  It  is  needless  to 
dwell  for  a  moment  on  the  advantage  of  possessing  a 
signal  commanding  ranges  such  as  these. 

The  explosion  of  substances  in  the  air,  after  having 
been  carried  to  a  considerable  elevation  by  rockets,  is 
a  familiar  performance.  In  1873,  moreover,  the  Board 
of  Trade  proposed  a  light-and-sound  rocket  as  a  signal 
of  distress,  which  proposal  was  subsequently  realized, 
but  in  a  form  too  elaborate  and  expensive  for  practical 
use.  The  idea  of  a  gun-cotton  rocket  fit  for  signalling 
in  fogs  is,  I  believe,  wholly  due  to  Sir  Richard 
Collinson,  the  Deputy  Master  of  the  Trinity  House. 
Thanks  to  the  skilful  aid  given  by  the  authorities  of 
Woolwich,  by  Mr.  Prentice,  and  Mr.  Brock,  that  idea 
is  now  an  accomplished  fact ;  a  signal  of  great  power, 
handiness,  and  economy,  being  thus  placed  at  the 
service  of  our  mariners.  Not  only  may  the  rocket  be 
applied  in  association  with  lighthouses  and  lightships, 
but  in  the  Navy  also  it  may  be  turned  to  important 
account.  Soon  after  the  loss  of  the  'Vanguard'  I 
ventured  to  urge  upon  an  eminent  naval  officer  the  de- 
sirability of  having  an  organized  code  of  fog-signals 
for  the  fleet.  He  shook  his  head  doubtingly,  and 
referred  to  the  difficulty  of  finding  room  for  signal  guns. 
The  gun-c5otton  rocket  completely  surmounts  this  diffi- 


EECENT  EXPERIMENTS  ON  FOG-SIGNALS.         275 

culty.  It  is  manipulated  with  ease  and  rapidity,  while 
its  discharges  may  be  so  grouped  and  combined  as  to 
give  a  most  important  extension  to  the  voice  of  the 
admiral  in  command.  It  is  needless  to  add  that  at  any 
point  upon  our  coasts,  or  upon  any  other  coast,  where 
its  establishment  might  be  desirable,  a  fog-signal 
station  might  be  extemporised  without  difficulty. 

I  have  referred  more  than  once  to  the  train  of  echoes 
which  accompanied  the  explosion  of  gun-cotton  in  free 
air,  speaking  of  them  as  similar  in  all  respects  to  those 
which  were  described  for  the  first  time  in  my  Report  on 
Fog-signals,  addressed  to  the  Corporation  of  Trinity 
House  in  1874.*  To  these  echoes  I  attached  a  funda- 
mental significance.  There  was  no  visible  reflecting 
surface  from  which  they  could  come.  On  some  days, 
with  hardly  a  cloud  in  the  air  and  hardly  a  ripple  on 
the  sea,  they  reached  a  magical  intensity.  As  far  as  the 
sense  of  hearing  could  judge,  they  came  from  the  body 
of  the  air  in  front  of  the  great  trumpet  which  produced 
them.  The  trumpet  blasts  were  five  seconds  in  dura- 
tion, but  long  before  the  blast  had  ceased  the  echoes 
struck  in,  adding  their  strength  to  the  primitive  note 
of  the  trumpet.  After  the  blast  had  ended  the  echoes 
continued,  retreating  further  and  further  from  the  point 
of  observation,  and  finally  dying  away  at  great  distances. 
The  echoes  were  perfectly  continuous  as  long  as  the  sea 
was  clear  of  ships,  '  tapering '  by  imperceptible  grada- 
tions into  absolute  silence.  But  when  a  ship  happened 
to  throw  itself  athwart  the  course  of  the  sound,  the  echo 
from  the  broadside  of  the  vessel  was  returned  as  a  shock 
which  rudely  interrupted  the  continuity  of  the  dying 
atmospheric  music. 

>  See  fUao  *  Pbilosopliical  Transactions '  for  1874,  p.  183, 


276  FKAGMENTS  OF  SCIENCE. 

These  echoes  have  been  ascribed  to  reflection  from 
the  crests  of  the  sea-waves.  But  this  hypothesis  is 
negatived  by  the  fact,  that  the  echoes  were  produced 
in  great  intensity  and  duration  when  no  waves  existed 
— when  the  sea,  in  fact,  was  of  glassy  smoothness.  It 
has  been  also  shown  that  the  direction  of  the  echoes 
depended  not  on  that  of  waves,  real  or  assumed,  but 
on  the  direction  of  the  axis  of  the  trumpet.  Causing 
that  axis  to  traverse  an  arc  of  210°,  and  the  trumpet 
to  sound  at  various  points  of  the  arc,  the  echoes  were 
always,  at  all  events  in  calm  weather,  returned  from 
that  portion  of  the  atmosphere  towards  which  the 
trumpet  was  directed.  They  could  not,  under  the 
circumstances,  come  from  the  glassy  sea;  while  both 
their  variation  of  direction  and  their  perfectly  con- 
tinuous fall  into  silence,  are  irreconcilable  with  the 
notion  that  they  came  from  fixed  objects  on  the  land. 
They  came  from  that  portion  of  the  atmosphere  into 
which  the  trumpet  poured  its  maximum  sound,  and  fell 
in  intensity  as  the  direct  sound  penetrated  to  greater 
atmospheric  distances. 

The  day  on  which  our  latest  observations  were  made 
was  particularly  fine.  Before  reaching  Dungeness,  the 
smoothness  of  the  sea  and  the  serenity  of  the  air  caused 
me  to  test  the  echoing  power  of  the  atmosphere.  A 
single  ship  lay  about  half  a  mile  distant  between  us  and 
the  land.  The  result  of  the  proposed  experiment  was 
clearly  foreseen.  It  was  this.  The  rocket  being  sent 
up,  it  exploded  at  a  great  height ;  the  echoes  retreated 
in  their  usual  fashion,  becoming  less  and  less  intense  as 
the  distances  of  the  invisible  surfaces  of  reflection  from 
the  observers  increased.  About  five  seconds  after  the 
explosion,  a  single  loud  shock  was  sent  back  to  us  from 
the  side  of  the  vessel  lying  between  us  and  the  land. 
Obliterated  for  a  moment  by  this  more  intense  echo. 


RECENT  EXPERIMENTS  ON  FOG-SIGNALS.  277 

the  aerial  reverberation  continued  its  retreat,  dying 
away  into  silence  in  two  or  three  seconds  afterwards.* 

I  have  referred  to  the  firing  of  an  8-oz.  rocket  from 
the  deck  of  the  '  Galatea'  on  March  8,  1877,  stating  the 
duration  of  its  echoes  to  be  seven  seconds.  Mr.  Prentice, 
who  was  present  at  the  time,  assured  me  that  in  his 
experiments  similar  echoes  had  been  frequently  heard 
of  more  than  twice  this  duration.  The  ranges  of  his 
sounds  alone  would  render  this  result  in  the  highest 
degree  probable. 

To  attempt  to  interpret  an  experiment  which  I  have 
not  had  an  opportunity  of  repeating,  is  an  operation  of 
some  risk ;  and  it  is  not  without  a  consciousness  of  this 
that  I  refer  here  to  a  result  announced  by  Professor 
Joseph  Henry,  which  he  considers  adverse  to  the  notion 
of  aerial  echoes.  He  took  the  trouble  to  point  the 
trumpet  of  a  syren  towards  the  zenith,  and  found  that 
when  the  syren  was  sounded  no  echo  was  returned. 
Now  the  reflecting  surfaces  which  give  rise  to  these 
echoes  are  for  the  most  part  due  to  differences  of  tem- 
perature between  sea  and  air.  If,  through  any  cause, 
the  air  above  be  chilled,  we  have  descending  streams — 
it  the  air  below  be  warmed,  we  have  ascending  streams 
as  the  initial  cause  of  atmospheric  flocculence.  A 
sound  proceeding  vertically  does  not  cross  the  streams, 
nor  impinge  upon  the  reflecting  surfaces,  as  does  a 
sound  proceeding  horizontally  across  them.  Aerial 
echoes,  therefore,  will  not  accompany  the  vertical  sound 
as  they  accompany  the  horizontal  one.  The  experiment, 
as  I  interpret  it,  is  not  opposed  to  the  theory  of  these 
echoes  which  I  have  ventured  to  enunciate.  But^  as  I 
have  indicated,  not  only  to  see  but  to  vary  such  an 

'  The  echoes  of  the  gun  fired  on  shore  this  day  were  very  brief ; 
those  of  the  12-oz.  gTin-cotton  rocket  were  12'  and  those  of  the  8-oa. 
cotton-powder  rocket  11"  in  duration. 


278  FKAGMENTS  OF  SCIENCE. 

experiment  is  a  necessary  prelude  to  grasping  its  full 
significance. 

In  a  paper  published  in  the  '  Philosophical  Trans- 
actions'  for  1876,  Professor  Osborne  Reynolds  refers 
to  these  echoes  in  the  following  terms  : — '  Without 
attempting  to  explain  the  reverberations  and  echoes 
which  have  been  observed,  I  will  merely  call  attention 
to  the  fact  that  in  no  case  have  I  heard  any  attending 
the  reports  of  the  rockets,^  although  they  seem  to  have 
been  invariable  with  the  guns  and  pistols.  These  facts 
suggest  that  the  echoes  are  in  some  way  connected  with 
the  direction  given  to  the  sound.  They  are  caused  by 
the  voice,  trumpets,  and  the  syren,  all  of  which  give 
direction  to  the  sound ;  but  I  am  not  aware  that  they 
have  ever  been  observed  in  the  case  of  a  sound  which  has 
no  direction  of  greatest  intensity.'  The  reference  to  the 
voice,  and  other  references  in  his  paper,  cause  me  to  think 
that,  in  speaking  of  echoes.  Professor  Osborne  Eeynolds 
and  myself  are  dealing  with  different  phenomena.  Be 
that  as  it  may,  the  foregoing  observations  render  it 
perfectly  certain  that  the  condition  as  to  direction  here 
laid  down  is  not  necessary  to  the  production  of  the 
echoes. 

There  is  not  a  feature  connected  with  the  aerial 
echoes  which  cannot  be  brought  out  by  experiments  in 
the  air  of  the  laboratory.  I  have  recently  made  the 
following  experiment : — A  rectangle,  x  y  (p.  279),  22 
inches  by  12,  was  crossed  by  twenty-three  brass  tubes 
(half  the  number  would  suffice  and  only  eleven  are 
shown  in  the  figure),  each  having  a  slit  along  it  from 
which  gas  can  issue.  In  this  way  twenty-three  low 
flat  flames  were  obtained.    A  sounding  reed  a  fixed  in  a 


i  These  carried  12  oz.  of  gunpowder,  which  has  been  found  by 
Ool.  Fraser  to  require  an  iron  case  to  produce  an  effective  explosion, 


EECENT  EXPERIMENTS  ON  FOG-SIGNALS.        279 


short  tube  was  placed  at  one  end  of  the  rectangle,  and 
a  '  sensitive  flame,'  ^  /,  at  some  distance  beyond  the 
other  end.  When  the  reed  sounded,  the  flame  in  front 
of  it  was  violently  agitated,  and  roared  boisterously. 
Turning  on  the  gas,  and  lighting  it  as  it  issued  from 
the  slits,  the  air  above  the  flames  became  so  hetero- 
geneous that  the  sensitive  flame  was  instantly  stilled, 
rising  from  a  height  of  6  inches  to  a  height  of  18 
inches.  Here  we  had  the  acoustic  opacity  of  the  air 
in  front  of  the  South  Foreland  strikingly  imitated.^ 
Turning  off  the  gas,  and  removing  the  sensitive  flame 
to  /',  some  distance  behind  the  reed,  it  burned  there 
tranquilly,   though    the   reed  was    sounding.      Again 


Fig.  9. 


lighting  the  gas   as   it   issued  from  the   brass  tubes, 
the  sound  reflected  from  the  heterogeneous  air  threw 

»  Fully  described  in  my  <  Lectures  on  Sound,*  3rd  edition,  p.  227. 
c  Lectures  on  Sound,  3rd  ed.,  p.  268. 


280  FRAGMENTS  OF  SCIENCE. 

the  sensitive  flame  into  violent  agitation.  Here  we 
had  imitated  the  aerial  echoes  heard  when  standing 
behind  the  syren-trumpet  at  the  South  Foreland.  The 
experiment  is  extremely  simple,  and  in  the  highest 
degree  impressive. 


The  explosive  rapidity  of  dynamite  marks  it  as  a 
substance  specially  suitable  for  the  production  of  sound. 
At  the  suggestion  of  Professor  Dewar,  Mr.  McEoberts 
has  carried  out  a  series  of  experiments  on  dynamite, 
with  extremely  promising  results.  Immediately  after 
the  delivery  of  the  foregoing  lecture  I  was  informed 
that  Mr.  Brock  proposed  the  employment  of  dynamite 
in  the  Collinson  rocket- 


XI. 

ON  THE  STUDY  OF  PHYSICS^ 

I  HOLD  in  my  hand  an  uncorrected  proof  of  the  sylla- 
bus of  this  course  of  lectures,  and  the  title  of 
the  present  lecture  is  there  stated  to  be  *  On  the  Inxport- 
ance  of  the  Study  of  Physics  as  a  Means  of  Education.' 
The  corrected  proof,  however,  contains  the  title  : — ^  On 
the  Importance  of  the  Study  of  Physics  as  a  Branch 
of  Education.'  Small  as  this  editorial  alteration  may 
seem,  the  two  words  suggest  two  radically  distinct 
modes  of  viewing  the  subject  before  us.  The  term 
Education  is  sometimes  applied  to  a  single  faculty  or 
organ,  and  if  we  know  wherein  the  education  of  a 
single  faculty  consists,  this  will  help  us  to  clearer 
notions  regarding  the  education  of  the  sum  of  all  the 
faculties,  or  of  the  mind.  When,  for  example,  we  speak 
of  the  education  of  the  voice,  what  do  we  mean? 
There  are  certain  membranes  at  the  top  of  the 
windpipe  which  throw  into  vibration  the  air  forced 
between  them  from  the  lungs,  thus  producing  musical 
sounds.  These  membranes  are,  to  some  extent,  under 
the  control  of  the  will,  and  it  is  found  that  they  can  be 
80  modified  by  exercise  as  to  produce  notes  of  a  clearer 
and  more  melodious  character.  This  exercise  we  call 
the  education  of  the  voice.     We  may  choose  for  our 

•  From  a  lecture  delivered  in  the  Koyal  Institution  of  Great 
Britain  in  the  Spring  o£  1854. 


282  FRAGMENTS  OF  SCIENCE. 

exercise  songs  new  or  old,  festive  or  solemn ;  the  edu- 
cation of  the  voice  being  the  object  aimed  at,  the  songs 
may  he  regarded  as  the  means  by  which  this  education 
is  accomplished.  I  think  this  expresses  the  state  of 
the  case  more  clearly  than  if  we  were  to  call  the  songs 
a  branch  of  education.  Regarding  also  the  education 
of  the  human  mind  as  the  improvement  and  develop- 
ment of  the  mental  faculties,  1  shall  consider  the  study 
of  Physics  as  a  means  towards  the  attainment  of  this 
end.  From  this  point  of  view,  I  degrade  Physics  into 
an  implement  of  culture,  and  this  is  my  deliberate 
design. 

The  term  Physics,  as  made  use  of  in  the  present 
Lecture,  refers  to  that  portion  of  natural  science  which 
lies  midway  between  astronomy  and  chemistry.  The 
former,  indeed,  is  Physics  applied  to  '  masses  of  enor- 
mous weight,'  while  the  latter  is  Physics  applied  to 
atoms  and  molecules.  The  subjects  of  Physics  proper 
are  therefore  those  which  lie  nearest  to  human  per- 
ception:— light  and  heat,  colour,  sound,  motion,  the 
loadstone,  electrical  attractions  and  repulsions,  thunder 
and  lightning,  rain,  snow,  dew,  and  so  forth.  Our 
senses  stand  between  these  phenomena  and  the  reasoning 
mind.  We  observe  the  fact,  but  are  not  satisfied  with 
the  mere  act  of  observation :  the  fact  must  be  accounted 
for — fitted  into  its  position  in  the  line  of  cause  and 
efifect.  Taking  our  facts  from  Nature  we  transfer 
them  to  the  domain  of  thouo:ht :  look  at  them,  compare 
them,  observe  their  mutual  relations  and  connexions, 
and  bringing  them  ever  clearer  before  the  mental  eye, 
finally  alight  upon  the  cause  which  unites  them.  This 
is  the  last  act  of  the  mind,  in  this  centripetal  direction — 
in  its  progress  from  the  multiplicity  of  facts  to  the 
central  cause  on  which  they  depend.  But,  having 
guessed  the  cause,  we  are  not  yet  contented.     We  set 


ON  THE  STUDY  OF  PHYSICa  283 

out  fro^  the  centre  and  travel  in  the  other  direction. 
If  the  guess  be  true,  certain  consequences  must  follow 
from  it,  and  we  appeal  to  the  law  and  testimony  of 
experiment  whether  the  thing  is  so.  Thus  is  the 
circuit  of  thought  completed, — from  without  inward, 
from  multiplicity  to  unity,  and  from  within  outward, 
from  unity  to  multiplicity.  In  thus  traversing  both 
ways  the  line  between  cause  and  effect,  all  our  reason- 
ing powers  are  called  into  play.  The  mental  effort 
involved  in  these  processes  may  be  compared  to  those 
exercises  of  the  body  which  invoke  the  co-operation  of 
every  muscle,  and  thus  confer  upon  the  whole  frame  the 
benefits  of  healthy  action. 

The  first  experiment  a  child  makes  is  a  physical 
experiment :  the  suction-pump  is  but  an  imitation  of 
the  first  act  of  every  new-born  infant.  Nor  do  I  think 
it  calculated  to  lessen  that  infant's  reverence,  or  to 
make  him  a  worse  citizen,  when  his  riper  experience 
shows  him  that  the  atmosphere  was  his  helper  in  ex- 
tracting the  first  draught  from  his  mother's  breast. 
The  child  grows,  but  is  still  an  experimenter  :  he  grasps 
at  the  moon,  and  his  failure  teaches  him  to  respect 
distance.  At  length  his  little  fingers  acquire  sufficient 
mechanical  tact  to  lay  hold  of  a  spoon.  He  thrusts 
the  instrument  into  his  mouth,  hurts  his  gums,  and 
thus  learns  the  impenetrability  of  matter.  He  lets  the 
spoon  fall,  and  jumps  with  delight  to  hear  it  rattle 
against  the  table.  The  experiment  made  by  accident 
is  repeated  with  intention,  and  thus  the  young  student 
receives  his  first  lessons  upon  sound  and  gravitation. 
There  are  pains  and  penalties,  however,  in  the  path  of 
the  enquirer :  he  is  sure  to  go  wrong,  and  Nature  is 
just  as  sure  to  inform  him  of  the  fact.  He  falls  down- 
stairs, burns  his  fingers,  cuts  his  hand,  scalds  his  tongue, 
and  in  this  way  learns  the  conditions  of  his  physical 


284  FRAGMENTS  OF  SCIENCE. 

well  being.  This  is  Nature's  way  of  proceeding,  and  it 
is  wonderful  what  progress  her  pupil  makes.  His 
enjoyments  for  a  time  are  physical,  and  the  con- 
fectioner's shop  occupies  the  foreground  of  human 
happiness  ;  but  the  blossoms  of  a  finer  life  are  already 
beginning  to  unfold  themselves,  and  the  relation  of 
cause  and  efifect  dawns  upon  the  boy.  He  begins  to 
see  that  the  present  condition  of  things  is  not  final, 
but  depends  upon  one  that  has  gone  before,  and  will  be 
succeeded  by  another.  He  becomes  a  puzzle  to  himself; 
and  to  satisfy  his  newly-awakened  curiosity,  asks  all 
manner  of  inconvenient  questions.  The  needs  and 
tendencies  of  human  nature  express  themselves  through 
these  early  yearnings  of  the  child.  As  thought  ripens, 
he  desires  to  know  the  character  and  causes  of  the 
phenomena  presented  to  his  observation ;  and  unless 
this  desire  has  been  granted  for  the  express  purpose  of 
having  it  repressed,  unless  the  attractions  of  natural 
phenomena  be  like  the  blush  of  the  forbidden  fruit, 
conferred  merely  for  the  purpose  of  exercising  our  self- 
denial  in  letting  them  alone ;  we  may  fairly  claim  for 
the  study  of  Physics  the  recognition  that  it  answers  to  an 
impulse  implanted  by  nature  in  the  constitution  of  man. 
A  few  days  ago,  a  Master  of  Arts,  who  is  still  a 
young  man,  and  therefore  the  recipient  of  a  modern 
education,  stated  to  me  tliat  until  he  had  reached  the 
age  of  twenty  years  he  had  never  been  taught  anything 
whatever  regarding  natural  phenomena,  or  natural  law. 
Twelve  years  of  his  life  previously  had  been  spent 
exclusively  among  the  ancients.  The  case,  I  regret  to 
say,  is  typical.  Now,  we  cannot,  without  prejudice  to 
humanity,  separate  the  present  from  the  past.  The 
nineteenth  century  strikes  its  roots  into  the  centuries 
gone  by,  and  draws  nutriment  from  them.  The  world 
cannot  afford  to  lose  the  record  of  any  great  deed  or 


ON  THE  STUDY  OF  PHYSICS.  285 

utterance ;  for  such  are  prolific  throughout  all  time. 
We  cannot  yield  the  companionship  of  our  loftier 
brothers  of  antiquity, — of  our  Socrates  and  Cato, — 
whose  lives  provoke  us  to  sympathetic  greatness  across 
the  interval  of  two  thousand  years.  As  long  as  the 
ancient  languages  are  the  means  of  access  to  the  ancient 
mind,  they  must  ever  be  of  priceless  value  to  humanity ; 
but  surely  these  avenues  might  be  kept  open  without 
making  such  sacrifices  as  that  above  referred  to,  uni- 
versal. We  have  conquered  and  possessed  ourselves  of 
continents  of  land,  concerning  which  antiquity  knew 
nothing ;  and  if  new  continents  of  thought  reveal  them- 
selves to  the  exploring  human  spirit,  shall  we  not 
possess  them  also  ?  In  these  latter  days,  the  study  of 
Physics  has  given  us  glimpses  of  the  methods  of  Nature 
which  were  quite  hidden  from  the  ancients,  and  we 
should  be  false  to  the  trust  committed  to  us,  if  we 
were  to  sacrifice  the  hopes  and  aspirations  of  the 
Present  out  of  deference  to  the  Past. 

The  bias  of  my  own  education  probably  manifests 
itself  in  a  desire  I  always  feel  to  seize  upon  every 
possible  opportunity  of  checking  my  assumptions  and 
conclusions  by  experience.  In  the  present  case,  it  is  true, 
your  own  consciousness  might  be  appealed  to  in  proof 
of  the  tendency  of  the  human  mind  to  inquire  into  the 
phenomena  presented  to  it  by  the  senses ;  but  I  trust  you 
will  excuse  me  if,  instead  of  doing  this,  I  take  advan- 
tage of  the  facts  which  have  fallen  in  my  way  through 
life,  referring  to  your  judgment  to  decide  whether  such 
facts  are  truly  representative  and  general,  and  not 
merely  individual  and  local. 

At  an  agricultural  college  in  Hampshire,  with  which 
I  was  connected  for  some  time,  and  which  is  now  con- 
verted into  a  school  for  the  general  education  of  youth, 
a  Society  was  formed  among  the  boys,  who  met  weekly 


286  FRAGMENTS  OF  SCIENCE. 

for  the  purpose  of  reading  reports  and  papers  upon 
various  subjects.  The  Society  had  its  president  and 
treasurer;  and  abstracts  of  its  proceedings  were  pub- 
lished in  a  little  monthly  periodical  issuing  from  the 
school  press.  One  of  the  most  remarkable  features 
of  these  weekly  meetings  was,  that  after  the  general 
business  had  been  concluded,  each  member  enjoyed  the 
right  of  asking  questions  on  any  subject  on  which  he 
desired  information.  The  questions  were  either  written 
out  previously  in  a  book,  or,  if  a  question  happened  to 
suggest  itself  during  the  meeting,  it  was  written  upon 
a  slip  of  paper  and  handed  in  to  the  Secretary,  who 
afterwards  read  all  the  questions  aloud.  A  number  of 
teachers  were  usually  present,  and  they  and  the  boys 
made  a  common  stock  of  their  wisdom  in  furnishing 
replies.  As  might  be  expected  from  an  assemblage  of 
eighty  or  ninety  boys,  varying  from  eighteen  to  eight 
years  old,  many  odd  questions  were  proposed.  To  the 
mind  which  loves  to  detect  in  the  tendencies  of  the 
young  the  instincts  of  humanity  generally,  such  ques- 
tions are  not  without  a  certain  philosophic  interest, 
and  I  have  therefore  thought  it  not  derogatory  to  the 
present  course  of  Lectures  to  copy  a  few  of  them,  and 
to  introduce  them  here.     They  run  as  follows  : — 

What  are  the  duties  of  the  Astronomer  Eoyal  ? 

What  is  frost  ? 

Why  are  thunder  and  lightning  more  frequent  in 
summer  than  in  winter  ? 

What  occasions  falling  stars  ? 

What  is  the  cause  of  the  sensation  called  '  pins  and 
needles '  ? 

What  is  the  cause  of  waterspouts  ? 

What  is  the  cause  of  hiccup  ? 

If  a  towel  be  wetted  with  water,  why  does  the  wet 
portion  become  darker  than  before  ? 


ON  THE  STUDY  OF  PHYSICS.  287 

What  is  meant  by  Lancashire  witches  ? 

Does  the  dew  rise  or  fall  ? 

What  is  the  principle  of  the  hydraulic  press  ? 

Is  there  more  oxygen  in  the  air  in  summer  than  in 
mnter  ? 

What  are  those  rings  which  we  see  round  the  gaa 
and  sun  ? 

What  is  thunder  ? 

How  is  it  that  a  black  hat  can  be  moved  by  forming 
round  it  a  magnetic  circle,  while  a  white  hat  remains 
stationary  ? 

What  is  the  cause  of  perspiration  ? 

Is  it  true  that  men  were  once  monkeys  ? 

What  is  the  difference  between  the  soul  and  the 

Is  it  contrary  to  the  rules  of  Vegetarianism  to  eat 
eggs? 

In  looking  over  these  questions,  which  were  wholly 
unprompted,  and  have  been  copied  almost  at  random 
from  the  book  alluded  to,  we  see  that  many  of  them 
are  suggested  directly  by  natural  objects,  and  are  not 
such  as  had  an  interest  conferred  on  them  by  previous 
culture.  Now  the  fact  is  beyond  the  boy's  control,  and 
80  certainly  is  the  desire  to  know  its  cause.  The  sole 
question  then  is,  whether  this  desire  is  to  be  gratified  or 
not.  Who  created  the  fact  ?  Who  implanted  the  desire  ? 
Certainly  not  man.  Who  then  will  undertake  to  place 
himself  between  the  desire  and  its  fulfilment,  and  pro- 
claim a  divorce  between  them  ?  Take,  for  example,  the 
case  of  the  wetted  towel,  which  at  first  sight  appears  to 
be  one  of  the  most  unpromising  questions  in  the  list. 
Shall  we  tell  the  proposer  to  repress  his  curiosity,  as 
the  subject  is  improper  for  him  to  know,  and  thus 
interpose  our  wisdom  to  rescue  the  boy  from  the  con- 
Bequences  of  a  wish  which  acts  to  his  prejudice  ?     Or, 


288  FKAGMENTS  OF  SCIENCE. 

recognising  the  propriety  of  the  question,  how  shall  we 
answer  it  ?  It  is  in[ipossible  to  answer  it  without 
reference  to  the  laws  of  optics — without  making  the 
boy  to  some  extent  a  natm-al  philosopher.  You  may 
say  that  the  effect  is  due  to  the  reflection  of  light 
at  the  common  surface  of  two  media  of  different 
refractive  indices.  But  this  answer  presupposes  on  the 
part  of  the  boy  a  knowledge  of  what  reflection  and 
refraction  are,  or  reduces  you  to  the  necessity  of 
explaining  them. 

On  looking  more  closely  into  the  matter,  we  find 
that  our  wet  towel  belongs  to  a  class  of  phenomena 
which  have  long  excited  the  interest  of  philosophers. 
The  towel  is  white  for  the  same  reason  that  snow  is 
white,  that  foam  is  white,  that  pounded  granite  or 
glass  is  white,  and  that  the  salt  we  use  at  table  is 
white.  On  quitting  one  medium  and  entering  another, 
a  portion  of  light  is  always  reflected,  but  on  this  condi- 
tion— the  media  must  possess  different  refractive  indices. 
Thus,  when  we  immerse  a  bit  of  glass  in  water,  light 
is  reflected  from  the  common  surface  of  both,  and  it  is 
this  light  which  enables  us  to  see  the  glass.  Bat  when 
a  transparent  solid  is  immersed  in  a  liquid  of  the  same 
refractive  index  as  itself,  it  immediately  disappears.  I 
remember  once  dropping  the  eyeball  of  an  ox  into 
water ;  it  vanished  as  if  by  magic,  with  the  exception 
of  the  crystalline  lens,  and  the  surprise  was  so  great 
as  to  cause  a  bystander  to  suppose  that  the  vitreous 
humour  had  been  instantly  dissolved.  This,  however, 
was  not  the  case,  and  a  comparison  of  the  refractive 
index  of  the  humour  with  that  of  water  cleared  up 
the  whole  matter.  The  indices  were  identical,  and 
hence  the  light  pursued  its  way  through  both  as  if  they 
formed  one  continuous  mass. 

In   the  case  of  snow,  powdered  quartz,  or  salt,  we 


ON   THE  STUDY  OF  PHYSICS.  289 

have  a  transparent  solid  mixed  with  air.  At  every 
transition  from  solid  to  air,  or  from  air  to  solid,  a  por- 
tion of  light  is  reflected,  and  this  takes  place  so  often 
that  the  light  is  wholly  intercepted.  Thus  from  the 
mixture  of  two  transparent  bodies  we  obtain  an  opaque 
one.  Now  the  case  of  the  towel  is  precisely  similar. 
The  tissue  is  composed  of  semi-transparent  vegetable 
fibres,  with  the  interstices  between  them  filled  with 
air ;  repeated  reflection  takes  place  at  the  limiting 
surfaces  of  air  and  fibre,  and  hence  the  towel  becomes 
opaque  like  snow  or  salt.  But  if  we  fill  the  interstices 
with  water,  we  diminish  the  reflection ;  a  portion  of 
the  light  is  transmitted,  and  the  darkness  of  the  towel 
is  due  to  its  increased  transparency.  Thus  the  deport- 
ment of  various  minerals,  such  as  hydrophane  and 
tabasheer,  the  transparency  of  tracing  paper  used  by 
engineers,  and  many  other  considerations  of  the  highest 
scientific  interest,  are  involved  in  the  simple  enquiry  of 
this  unsuspecting  little  boy. 

Again,  take  the  question  regarding  the  rising  or 
falling  of  the  dew — a  question  long  agitated,  and  finally 
set  at  rest  by  the  beautiful  researches  of  Wells.  I  do 
not  think  that  any  boy  of  average  intelligence  will  be 
satisfied  with  the  simple  answer  that  the  dew  falls. 
He  will  wish  to  learn  how  you  know  that  it  falls,  and, 
if  acquainted  with  the  notions  of  the  middle  ages,  he 
may  refer  to  the  opinion  of  Father  Laurus,  that  a  goose 
egg  filled  in  the  morning  with  dew  and  exposed  to  the 
sun,  will  rise  like  a  balloon — a  swan's  egg  being  better 
for  the  experiment  than  a  goose  egg.  It  is  impossible 
to  give  the  boy  a  clear  notion  of  the  beautiful  pheno- 
menon to  which  his  question  refers,  without  first 
making  him  acquainted  with  the  radiation  and  con- 
duction of  heat.  Take,  for  example,  a  blade  of  grass, 
from  which  one  of  these  orient  pearls  is  depending. 


290  FRAGMENTS  OF  SCIENCE. 

During  the  day  the  grass,  and  the  earth  beneath  it, 
possess  a  certain  amount  of  warmth  imparted  by  the 
sun ;  during  a  serene  night,  heat  is  radiated  from  the 
surface  of  the  grass  into  space,  and  to  supply  the  loss, 
there  is  a  flow  of  heat  from  the  earth  to  the  blade. 
Thus  the  blade  loses  heat  by  radiation,  and  gains  heat 
by  conduction.  Now,  in  the  case  before  us,  the  power 
of  radiation  is  great,  whereas  the  power  of  conduction 
is  small ;  the  consequence  is  that  the  blade  loses  more 
than  it  gains,  and  hence  becomes  more  and  more 
refrigerated.  The  light  vapour  floating  around  the 
surface  so  cooled  is  condensed  upon  it,  and  there  accu- 
mulates to  form  the  little  pearly  globe  which  we  call  a 
dew-drop. 

Thus  the  boy  finds  the  simple  and  homely  fact 
which  addressed  his  senses  to  be  the  outcome  and  flower 
of  the  deepest  laws.  The  fact  becomes,  in  a  measure, 
sanctified  as  an  object  of  thought,  and  invested  for  him 
with  a  beauty  for  evermore.  He  thus  learns  that 
things  which,  at  first  sight,  seem  to  stand  isolated  and 
without  apparent  brotherhood  in  Nature  are  organically 
united,  and  finds  the  detection  of  such  analogies  a  source 
of  perpetual  delight.  To  enlist  pleasure  on  the  side  of 
intellectual  performance  is  a  point  of  the  utmost  im- 
portance ;  for  the  exercise  of  the  mind,  like  that  of  the 
body,  depends  for  its  value  upon  the  spirit  in  which  it 
is  accomplished.  Every  physician  knows  that  some- 
thing more  than  mere  mechanical  motion  is  compre- 
hended under  the  idea  of  healthful  exercise — that, 
indeed,  being  most  healthful  which  makes  us  forget  all 
ulterior  ends  in  the  mere  enjoyment  of  it.  What,  for 
example,  could  be  substituted  for  the  action  of  the 
playground,  where  the  boy  plays  for  the  mere  love  of 
playing,  and  without  reference  to  physiological  laws; 
while   kindly   Nature   accomplishes   her   ends    uncon- 


ON  THE  STUDY  OF  PHYSICS.  291 

Bciously,  and  makes  his  very  indifference  beneficial  to 
him.  You  may  have  more  systematic  motions,  you  , 
may  devise  means  for  the  more  perfect  traction  of  eaoh 
particular  muscle,  but  you  cannot  create  the  joy  and 
gladness  of  the  game,  and  where  these  are  absent,  tlie 
charm  and  the  health  of  the  exercise  are  gone.  The 
case  is  similar  with  the  education  of  the  mind. 

The  study  of  Physics,  as  already  intimated,  consists 
of  two   processes,  which  are   complementary  to  each 
other — the  tracing  of  facts  to  their  causes,  and  the 
logical  advance  from  the  cause  to  the  fact.     In  the 
former  process,  called  induction,  certain  moral  qualities 
come  into  play.    The  first  condition  of  success  is  patient 
industry,  an  honest  receptivity,  and  a  willingness  to 
abandon  all  preconceived  notions,  however  cherished,  if 
they  be  found  to  contradict  the  truth.     Believe  me,  a 
self-renunciation  which  has  something  lofty  in  it,  and 
of  which  the  world  never  hears,  is  often  enacted  in  the 
private  experience  of  the  true  votary  of  science.     And 
if  a  man  be  not  capable  of  this  self-renunciation — this 
loyal  surrender  of  himself  to  Nature  and  to  fact,  he 
lacks,  in  my  opinion,  the  first  mark  of  a  true  philo- 
sopher.    Thus  the  earnest  prosecutor  of  science,  who 
does  not  work  with  the  idea  of  producing  a  sensation 
in   the   world,  who   loves   the  truth   better  than  the 
transitory  blaze  of  to-day  tj  fame,  who  comes  to  his  task 
with  a  single  eye,  finds  in  that  task  an  indirect  means 
of  the  highest  moral  culture.     And  although  the  virtue 
of  the  act  depends  upon  its  privacy,  this  sacrifice  of 
self,  this  upright  determination  to  accept  the  truth,  no 
matter  how  it  may  present  itself — even  at  the  hands  of 
a  scientific  foe,  if  necessary — carries  with  it  its  own 
reward.     When  prejudice  is  put  under  foot  and  the 
stains  of  personal  bias  have  been  washed  away — when  a 
man  consents  to  lay  aside  his  vanity  and  to  become 


202  FRAaMENTS  OF  SCIENCE. 

Nature's  organ — his  elevation  is  the  instant  consequence 
of  his  humility.  I  should  not  wonder  if  my  remaiks 
provoked  a  smile,  for  they  seem  to  indicate  that  I 
regard  the  man  of  science  as  a  heroic,  if  not  indeed 
an  angelic,  character;  and  cases  may  occur  to  you 
which  indicate  the  reverse.  You  may  point  to  the 
quarrels  of  scientific  men,  to  their  struggles  for  priority, 
to  that  unpleasant  egotism  which  screams  around  its 
little  property  of  discovery  like  a  scared  plover  about 
its  young.  I  will  not  deny  all  this ;  but  let  it  be  set 
down  to  its  proper  account,  to  the  weakness — or,  if 
you  will — to  the  selfishness  of  Man,  but  not  to  the 
charge  of  Physical  Science. 

The  second  process  in  physical  investigation  is  de- 
duction, or  the  advance  of  the  mind  from  fixed  prin- 
ciples to  the  conclusions  which  flow  from  them.  The 
rules  of  logic  are  the  formal  statement  of  this  process, 
which,  however,  was  practised  by  every  healthy  mind 
before  ever  such  rules  were  written.  In  the  study  of 
Physics,  induction  and  deduction  are  perpetually  wedded 
to  each  other.  The  man  observes,  strips  facts  of  their 
peculiarities  of  form,  and  tries  to  unite  them  by  their 
essences ;  having  effected  this,  he  at  once  deduces,  and 
thus  checks  his  induction.  Here  the  grand  difference 
between  the  methods  at  present  followed,  and  those  of 
the  ancients,  becomes  manifest.  They  were  one-sided 
in  these  matters :  they  omitted  the  process  of  induction, 
and  substituted  conjecture  for  observation.  They  could 
never,  therefore,  fulfil  the  mission  of  Man  to  *  replenish 
the  earth,  and  subdue  it.'  The  subjugation  of  Nature 
is  only  to  be  accomplished  by  the  penetration  of  her 
secrets  and  the  patient  mastery  of  her  laws.  This  not 
only  enables  us  to  protect  ourselves  from  the  hostile  action 
of  natural  forces,  but  makes  them  our  slaves.  By  the 
study  of  Physics  we  have  indeed  opened  to  us  treasuries 


ON  THE  STUDY  OF  PHYSICS.  293 

of  power  of  which  antiquity  never  dreamed.  But  while 
we  lord  it  over  Matter,  we  have  thereby  become  better 
acquainted  with  the  laws  of  Mind ;  for  to  the  mental 
philosopher  the  study  of  Physics  furnishes  a  screen 
against  which  the  human  spirit  projects  its  own  image^ 
and  thus  becomes  capable  of  self-inspection. 

Thus,  then,  as  a  means  of  intellectual  culture,  the 
study  of  Physics  exercises  and  sharpens  observation  :  it 
brings  the  most  exhaustive  logic  into  play :  it  compares, 
abstracts,  and  generalizes,  and  provides  a  mental  scenery 
appropriate  to  these  processes.  The  stricte.-t  precision 
of  thought  is  everywhere  enforced,  and  prudence,  fore- 
sight, and  sagacity  are  demanded.  By  its  appeals  to 
experiment,  it  continually  checks  itself,  and  thus  walks 
on  a  foundation  of  facts.  Hence  the  exercise  it  invokes 
does  not  end  in  a  mere  game  of  intellectual  gymnastics, 
such  as  the  ancients  delighted  in,  but  tends  to  the 
mastery  of  Nature.  This  gradual  conquest  of  the  ex- 
ternal world,  and  the  consciousness  of  augmented 
strength  which  accompanies  it,  render  the  study  of 
Physics  as  delightful  as  it  is  important. 

With  regard  to  the  effect  on  the  imagination,  certain 
it  is  that  the  cool  results  of  physical  induction  furnish 
conceptions  which  transcend  the  most  daring  flights  of 
that  faculty.  Take  for  example  the  idea  of  an  all- 
pervading  ether  which  transmits  a  tingle,  so  to  speak, 
to  the  finger  ends  of  the  universe  every  time  a  street 
lamp  is  lighted.  The  invisible  billows  of  this  ether  can 
be  measured  with  the  same  ease  and  certainty  as  that 
with  which  an  engineer  measures  a  base  and  two  angles, 
and  from  these  finds  the  distance  across  the  Thames. 
Now  it  is  to  be  confessed  that  there  may  be  just  as  little 
poetry  in  the  measurement  of  an  ethereal  undulation 
as  in  'that  of  the  river ;  for  the  intellect,  during  the  acts 
of  measurement  and  calculation,  destroys  those  notions 


294  FEAGMENTS  OF  SCIENCE. 

of  size  which  appeal  to  the  poetic  sense.  It  is  a  mis- 
take to  suppose,  with  Dr.  Young,  that 

An  undevout  astronomer  is  mad  ; 

there  being  no  necessary  connexion  between  a  devout 
state  of  mind  and  the  observations  and  calculations  of 
a  practical  astronomer.  It  is  not  until  the  man 
withdraws  from  his  calculation,  as  a  painter  from  his 
work,  and  thus  realizes  the 'great  idea  on  which  he  has 
been  engaged,  that  imagination  and  wonder  are  excited. 
There  is,  I  admit,  a  possible  danger  here.  If  the  arith- 
metical processes  of  science  be  too  exclusively  pursued, 
they  may  impair  the  imagination,  and  thus  the  study 
of  Physics  is  open  to  the  same  objection  as  philological, 
theological,  or  political  studies,  when  carried  to  excess. 
But  even  in  this  case,  the  injury  done  is  to  the  in- 
vestigator himself:  it  does  not  reach  the  mass  of 
mankind.  Indeed,  the  conceptions  furnished  by  his 
cold  unimaginative  reckonings  may  furnish  themes  for 
the  poet,  and  excite  in  the  highest  degree  that  senti- 
ment of  wonder  which,  notwithstanding  all  its  foolish 
vagaries,  table-turning  included,  I,  for  my  part,  should 
be  sorry  to  see  banished  from  the  world. 

I  have  thus  far  dwelt  upon  the  study  of  Physics  as 
an  agent  of  intellectual  culture ;  but  like  other  things 
in  Nature,  this  study  subserves  more  than  a  single  end. 
The  colours  of  the  clouds  delight  the  eye,  and,  no 
doubt,  accomplish  moral  purposes  alpo,  but  the  self- 
same clouds  hold  within  their  fleeces  the  moisture  by 
which  our  fields  are  rendered  fruitful.  The  sunbeams 
excite  our  interest  and  invite  our  invest  location :  but 
they  also  extend  their  beneficent  influences  to  our  fruits 
and  corn,  and  thus  accomplish,  not  only  intellectual 
ends,  but  minister,  at  the  same  time,  to  our  material 
necessities.      And   so   it   is   with    scientific    research. 


ON  THE  STUDY  OF  PHYSICS.  295 

While  the  love  of  science  is  a  sufiBcient  incentive  to 
the  pursuit  of  science,  and  the  investigator,  in  the 
prosecution  of  his  enquiries,  is  raised  above  all  material 
considerations,  the  results  of  his  labours  may  exercise  a 
potent  influence  upon  the  physical  condition  of  the 
communitjy.  This  is  the  arrangement  of  Nature,  and 
not  that  of  the  scientific  investigator  himself;  for  he 
usually  pursues  his  object  without  regard  to  its  practical 
applications. 

And  let  him  who  is  dazzled  by  such  applications — 
who  sees  in  the  steam-engine  and  the  electric  telegraph 
the  highest  embodiment  of  human  genius  and  the  only 
legitimate  object  of  scientific  research,  beware  of  pre- 
scribing conditions  to  the  investigator.  Let  him  be- 
ware of  attempting  to  substitute  for  that  simple  love 
with  which  the  votary  of  science  pursues  his  task,  the 
calculations  of  what  he  is  pleased  to  call  utility.  The 
professed  utilitarian  is  unfortunately,  in  most  cases,  the 
very  last  man  to  see  the  occult  sources  from  which 
useful  results  are  derived.  He  admires  the  flower,  but 
is  ignorant  of  the  conditions  of  its  growth.  The  scien- 
tific man  must  approach  Nature  in  his  own  way ;  for 
if  you  invade  his  freedom  by  your  so-called  practical 
considerations,  it  may  be  at  the  expense  of  those 
qualities  on  which  his  success  as  a  discoverer  depends. 
Let  the  self-styled  practical  man  look  to  those  frpm 
the  fecundity  of  whose  thought  he,  and  thousands  like 
him,  have  sprung  into  existence.  Were  they  inspired 
in  their  first  enquiries  by  the  calculations  of  utility  ? 
Not  one  of  them.  They  were  often  forced  to  live  low 
and  lie  hard,  and  to  seek  compensation  for  their 
penury  in  the  delight  which  their  favouiite  pursuits 
aflbrded  them.  In  the  words  of  one  well  qualified  to 
epeak  upon  this  subject,  '  I  say  not  merely  look  at  the 
pittance  ot   men  like  John  Dalton,  or  the  voluntary 

20 


296  FRAGMENTS  OF  SCIENCE. 

starvaUon  of  the  late  Graff;  but  compare  what  is 
considered  as  competency  or  affluence  by  your  Faradays, 
Liebigs,  and  Herschels,  with  the  expected  results  of  a 
life  of  successful  commercial  enterprise :  then  compare 
the  amount  of  mind  put  forth,  the  work  done  for  society 
in  either  case,  and  you  will  be  constrained  to  allow  that 
the  former  belong  to  a  class  of  workers  who,  properly 
speaking,  are  not  paid,  and  cannot  be  paid  for  their 
work,  as  indeed  it  is  of  a  sort  to  which  no  payment 
could  stimulate.' 

But  while  the  scientific  investigator,  standing  upon 
the  frontiers  of  human  knowledge,  and  aiming  at 
the  conquest  of  fresh  soil  from  the  surroimding  region 
of  the  unknown,  makes  the  discovery  of  truth  his  ex- 
clusive object  for  the  time,  he  cannot  but  feel  the 
deepest  interest  in  the  practical  application  of  the 
truth  discovered.  There  is  something  ennobling  in  the 
triumph  of  Mind  over  Matter.  Apart  even  from  its 
uses  to  society,  there  is  something  elevating  in  the  idea 
of  Man  having  tamed  that  wild  force  which  flashes 
through  the  telegraphic  wire,  and  made  it  the  minister 
of  his  will.  Our  attainments  in  these  directions  appear 
to  be  commensurate  with  our  needs.  We  had  already 
subdued  horse  and  mule,  and  obtained  from  them  all 
the  service  which  it  was  in  their  power  to  render :  we 
must  either  stand  still,  or  find  more  potent  agents  to 
execute  our  purposes.  At  this  point  the  steam-engine 
appears.  These  are  still  new  things ;  it  is  not  long 
since  we  struck  into  the  scientific  methods  which  have 
produced  these  results.  We  cannot  for  an  instant 
regard  them  as  the  final  achievements  of  Science,  but 
rather  as  an  earnest  of  what  she  is  yet  to  do.  They 
mark  our  first  great  advances  upon  the  dominion  ot 
Nature.  Animal  strength  fails,  but  here  are  the  forces 
which  hold  the  world  together,  and  the  instincts  and 


ON  THE  STUDY  OF  PHYSICS.  297 

successes  of  Man  assure  him  that  these  forces  are  his 
when  he  is  wise  enough  to  command  them. 

As  an  instrument  of  intellectual  culture,  the  study 
of  Physics  is  profitable  to  all :  as  hearing  upon  special 
functions,  its  value,  though  not  so  great,  is  still  more 
tangible.  Why,  for  example,  should  Members  of  Par- 
liament bo  ignorant  of  the  subjects  concerning  which 
they  are  called  upon  to  legislate?  In  this  land  of 
practical  physics,  why  should  they  be  unable  to  form 
an  independent  opinion  upon  a  physical  question? 
Why  should  the  member  of  a  parliamentary  committee 
be  left  at  the  mercy  of  interested  disputants  when  a 
scientific  question  is  discussed,  until  he  deems  the  nap 
a  blessing  which  rescues  him  from  the  bewilderments 
of  the  committee-room?  The  education  which  does 
not  supply  the  want  here  referred  to,  fails  in  its  duty 
to  England.  With  regard  to  our  working  people,  in 
the  ordinary  sense  of  the  term  working,  the  study  of 
Physics  would,  I  imagine,  be  profitable,  not  only  as  a 
means  of  intellectual  culture,  but  also  as  a  moral 
influence  to  woo  them  from  pursuits  which  now 
degrade  them.  A  man's  reformation  oftener  depends 
upon  the  indirect,  than  upon  the  direct  action  of  the 
will.  The  will  must  be  exerted  in  the  choice  of  em- 
ployment which  shall  break  the  force  of  temptation 
by  erecting  a  barrier  against  it.  The  drunkard,  for 
example,  is  in  a  perilous  condition  if  he  content  himself 
merely  with  saying,  or  swearing,  that  he  will  avoid  strong 
drink.  His  thoughts,  if  not  attracted  by  another  force, 
•will  revert  to  the  public-house,  and  to  rescue  him  per- 
manently from  this,  you  must  give  him  an  equivalent. 

By  investing  the  objects  of  hourly  intercourse  with 
an  interest  which  prompts  reflection,  new  enjoyments 
would  be  opened  to  the  working  man,  and  every  one  of 
these  would  be  a  point  of  force  to  protect  him  against 


298  FRAGMENTS  OF  SCIENCE. 

temptation.  Besides  this,  our  factories  and  our  foun- 
dries present  an  extensive  field  of  observation,  and 
were  those  who  work  in  them  rendered  capable,  by 
previous  culture,  of  observing  what  they  aee^  the 
results  might  be  incalculable.  Who  can  say  what 
intellectual  Samsons  are  at  the  present  moment  toiling 
with  closed  eyes  in  the  mills  and  forges  of  Manchester 
and  Birmingham?  Grrant  these  Samsons  sight,  and 
you  multiply  the  chances  of  discovery,  and  with  them 
the  prospects  of  national  advancement.  In  our  multi- 
tudinous technical  operations  we  are  constantly  playing 
with  forces  our  ignorance  of  which  is  often  the  cause  of 
our  destruction.  There  are  agencies  at  work  in  a 
locomotive  of  which  the  maker  of  it  probably  never 
dreamed,  but  which  nevertheless  may  be  sufficient 
to  convert  it  into  an  engine  of  death.  When  we 
reflect  on  the  intellectual  condition  of  the  people  who 
work  in  our  coal  mines,  those  terrific  explosions  which 
occur  from  time  to  time  need  not  astonish  us.  If  these 
men  possessed  sufficient  physical  knowledge,  from  the 
operatives  themselves  would  probably  emanate  a  system 
by  which  these  shocking  accidents  might  be  avoided. 
Possessed  of  the  knowledge,  their  personal  interests 
would  furnish  the  necessary  stimulus  to  its  practical 
application,  and  thus  two  ends  would  be  served  at  the 
same  time — the  elevation  of  the  men  and  the  diminu- 
tion of  the  calamity. 

Before  the  present  Course  of  Lectures  was  publicly 
announced,  I  had  many  misgivings  as  to  the  propriety 
of  my  taking  a  part  in  them,  thinking  that  my  place 
might  be  better  filled  by  an  older  and  more  experienced 
man.  To  my  experience,  however,  such  as  it  was,  I 
resolved  to  adhere,  and  I  have  therefore  described 
things  as  they  revealed  themselves  to  my  own  eyes,  and 
b.*v§  been  enacted  in  mj  own  limited  practice.     There 


ON  THE  STUDY  OF  PHYSICS.  299 

is  one  mind  common  to  us  all ;  and  the  true  expression 
of  this  mind,  even  in  small  particulars,  will  attest 
itself  by  the  response  which  it  calls  forth  in  the 
convictions  of  my  hearers.  I  ask  your  permission  to 
proceed  a  little  further  in  this  fashion,  and  to  refer  to 
a  fact  or  two  in  addition  to  those  already  cited,  which 
presented  themselves  to  my  notice  durinj^  my  brief 
career  as  a  teacher  in  the  college  already  alluded  to. 
The  facts,  though  extremely  humble,  and  deviating  in 
some  slight  degree  from  the  strict  subject  of  the  present 
discourse,  may  yet  serve  to  illustrate  an  educational 
principle. 

One  of  the  duties  which  fell  to  my  share  was  the 
instruction  of  a  class  in  mathematics,  and  I  usually 
found  that  Euclid  and  the  ancient  geometry  generally, 
when  properly  and  sympathetically  addressed  to  the 
understanding,  formed  a  most  attractive  study  for 
youth.  But  it  was  my  habitual  practice  to  with- 
draw the  boys  from  the  routine  of  the  book,  and 
to  appeal  to  their  self- power  in  the  treatment  of 
questions  not  comprehended  in  that  routine.  At 
first,  the  change  from  the  beaten  track  usually  excited 
aversion :  the  youth  felt  like  a  child  amid  strangers ; 
but  in  no  single  instance  did  this  feeling  continue. 
When  utterly  disheartened,  I  have  encouraged  the 
boy  by  the  anecdote  of  Newton,  where  he  attributes 
the  difference  between  him  and  other  men,  mainly 
to  his  own  patience  ;  or  of  Mirabeau,  when  he  ordered 
his  servant,  who  had  stated  something  to  be  impos- 
sible, never  again  to  use  that  blockhead  of  a  word. 
Thus  cheered,  the  boy  has  retiuned  to  his  task  with 
a  smile,  which  perhaps  had  something  of  doubt  in  it, 
but  which,  nevertheless,  evinced  a  resolution  to  try 
again.  I  have  seen  his  eye  brighten,  and,  at  length, 
with  a  pleasure  of  which  the  ecstasy  of  Archimedes  was 


300  FRAGMENTS  OF  SCIENCE. 

but  a  simple  expansion,  heard  him  exclaim,  '  I  have  it, 
sir.'  The  consciousness  of  self-power,  thus  awakened, 
was  of  immense  value ;  and,  animated  by  it,  the  pro- 
gress of  the  class  was  astonishing.  It  was  often  my 
custom  to  give  the  boys  the  choice  of  pursuing  their 
propositions  in  the  book,  or  of  trying  their  strength 
at  others  not  to  be  found  there.  Never  in  a 
single  instance  was  the  book  chosen.  I  was  ever 
ready  to  assist  when  help  was  needful,  but  my  offers 
of  assistance  were  habitually  declined.  The  boys 
had  tasted  the  sweets  of  intellectual  conquest  and 
demanded  victories  of  their  own.  Their  diagrams 
were  scratched  on  the  walls,  cut  into  the  beams  upon 
the  playground,  and  numberless  other  illustrations  were 
afforded  of  the  living  interest  they  took  in  the  subject. 
For  my  own  part,  as  far  as  experience  in  teaching  goes, 
I  was  a  mere  fledgling--knowingnothingof  the  rules  of 
pedagogics,  as  the  Grermans  name  it ;  but  adhering  to 
the  spirit  indicated  at  the  commencement  of  this  dis- 
course, and  endeavouring  to  make  geometry  a  means 
rather  than  a  branch  of  education.  The  experiment 
was  successful,  and  some  of  the  most  delightful  hours 
of  my  existence  have  been  spent  in  marking  the  vigorous 
and  cheerful  expansion  of  mental  power,  when  appealed 
to  in  the  manner  here  described. 

Our  pleasure  was  enhanced  when  we  applied  our 
mathematical  knowledge  to  the  solution  of  physical 
problems.  Many  objects  of  hourly  contact  had  thus  a 
new  interest  and  significance  imparted  to  them.  The 
swing,  the  see-saw,  the  tension  of  the  giant- stride  ropes, 
the  fall  and  rebound  of  the  football,  the  advantage  of  a 
small  boy  over  a  large  one  when  turning  short,  par- 
ticularly in  slippy  weather ;  all  became  subjects  of 
investigation.  A  lady  stands  before  a  looking-glass,  of 
her  own  height ;  it  was  required  to  know  how  much  of 


ON  THE  STXJDY  OF  PHYSICS.  301 

the  glass  was  really  useful  to  her  ?     We  learned  with 
pleasure   the   economic  fact  that  she  might  dispense 
with  the  lower  half  and  see  her  whole  figure  notwith- 
standing.     It  was  also  pleasant  to  prove  by  mathe- 
matics, and   verify  by   experiment,  that   the   angular 
velocity  of  a  reflected  beam  is  twice  that  of  the  mirror 
which  reflects  it.     From  the  hum  of  a  bee  we  were 
able  to  determine  the  number  of  times  the  insect  flaps 
its  wings  in  a  second.     Following  up  our  researches 
upon  the  pendulum,  we  learned  how  Colonel  Sabine 
had  made  it  the  means  of  determining  the  figure  of 
the  earth ;  and  we  were  also  startled  by  the  inference 
which  the  pendulum  enabled  us  to  draw,  that  if  the 
diurnal  velocity  of  the  earth  were  seventeen  times  its 
present  amount,  the  centrifugal  force  at  the  equator 
would  be  precisely  equal  to  the  force  of  gravitation,  so 
that  an  inhabitant  of  those  regions  would  then  have 
the  same  tendency  to  fail  upwards  as  downwards.     All 
these  things  were  sources  of  wonder  and  delight  to  us : 
and  when  we  remembered  that  we  were  gifted  with  the 
powers  which  had  reached  such  results,  and  that  the 
same  great  field  was  ours  to  work  in,  our  hopes  arose 
that  at  some  future  day  we  might  possibly  push  the 
subject  a  little  further,  and  add  our  own  victories  to 
the  conquests  already  won. 

I  ought  to  apologise  to  you  for  dwelling  so  long 
upon  this  subject ;  but  the  days  spent  among  these 
young  philosophers  made  a  deep  impression  on  me.  I 
learned  among  them  something  of  myself  and  of  human 
nature,  and  obtained  some  notion  of  a  teacher's  vocation. 
If  there  be  one  profession  in  England  of  paramount 
importance,  I  believe  it  to  be  that  of  the  schoolmaster ; 
and  if  there  be  a  position  where  selfishness  and  in- 
competence do  most  serious  mischief,  by  lowering  the 
moral  tone  and  exciting  irreverence  and  cunning  where 


302  MIAGMENTS  OF  SCIENCE. 

reverence  and  noble  truthfulness  ought  to  be  the 
feelings  evoked,  it  is  that  of  the  principal  of  a  school. 
When  a  man  of  enlarged  heart  and  mind  comes  among 
boys, — when  he  allows  his  spirit  to  stream  through 
them,  and  observes  the  operation  of  his  own  character 
evidenced  in  the  elevation  of  theirs, — it  woul  1  be  idle 
to  talk  of  the  position  of  such  a  man  being  honourable. 
It  is  a  blessed  position.  The  man  is  a  blessing  to 
himself  and  to  all  around  him.  Such  men,  I  believe, 
are  to  be  found  in  England,  and  it  behoves  those  who 
busy  themselves  with  the  mechanics  of  education  at 
the  present  day,  to  seek  them  out.  For  no  matter 
what  means  of  culture  may  be  chosen,  whether  physical 
or  philological,  success  must  ever  mainly  depend  upon 
the  amount  of  life,  love,  and  earnestness,  which  the 
teacher  himself  brings  with  him  to  his  vocation. 

Let  me  again,  and  finally,  remind  you  that  the  claims 
of  that  science  which  finds  in  me  to-day  its  unripened 
advocate,  are  those  of  the  logic  of  Nature  upon  the  reason 
of  her  child — that  its  disciplines,  as  an  agent  of  culture, 
are  based  upon  the  natural  relations  subsisting  between 
Man  and  the  universe  of  which  he  forms  a  part.  On 
the  one  side,  we  have  the  apparently  lawless  shifting  of 
phenomena;  on  the  other  side,  mind,  which  requires 
law  for  its  equilibrium,  and  through  its  own  indestruc- 
tible instincts,  as  well  as  through  the  teachings  of 
experience,  knows  that  these  phenomena  are  reducible 
to  law.  To  chasten  this  apparent  chaos  is  a  problem 
which  man  has  set  before  him.  The  world  was  built  in 
order:  and  to  us  are  trusted  the  will  and  power  to 
discern  its  harmonies,  and  to  make  them  the  lessons  of 
our  lives.  From  the  cradle  to  the  grave  we  are  sur- 
rounded with  objects  which  provoke  inquiry.  Descend- 
ing for  a  moment  from  this  high  plea  to  considerations 
which  lie  closer  to  us  as  a  nation — as  a  land  of  gas 


ON  THE  STUDY  OF  PHYSICS.  303 

and  furnaces,  of  steam  and  electricity :  as  a  land  which 
science,  practically  applied,  has  made  great  in  peace 
and  mighty  in  war : — I  ask  you  whether  this  '  land  of 
old  and  just  renown  *  has  not  a  right  to  expect  from  her 
institutions  a  culture  more  in  accordance  with  her 
present  needs  than  that  supplied  by  declension  and 
conjugation?  And  if  the  tendency  should  be  to  lower 
the  estimate  of  science,  by  regarding  it  exclusively  as 
the  instrument  of  material  prosperity,  let  it  be  the 
high  mission  of  our  universities  to  furnish  the  proper 
counterpoise  by  pointing  out  its  nobler  uses — lifting 
the  national  mind  to  the  contemplation  of  it  as  the  last 
development  of  that '  increasing  purpose  '  which  runs 
through  the  ages  and  widens  the  thoughts  of  men. 


304  FRAGMENTS  OF  SCIENCE. 


XII. 

ON  CRYSTALLINE  AND  SLATY  CLEAVAGE.^ 

WHEN  the  student  of  physical  science  has  to  investi- 
gate the  character  of  any  natural  force,  his  first 
care  must  be  to  purify  it  from  the  mixtiure  of  other  forces, 
and  thus  study  its  simple  action.  If,  for  example,  he 
wishes  to  know  how  a  mass  of  liquid  would  shape  itself  if 
at  Hberty  to  follow  the  bent  of  its  own  molecular  forces, 
he  must  see  that  these  forces  have  free  and  undisturbed 
exercise.  We  might  perhaps  refer  him  to  the  dew- 
drop  for  a  solution  of  the  question ;  but  here  we  have 
to  do,  not  only  with  the  action  of  the  molecules  of 
the  liquid  upon  each  other,  but  also  with  the  action  of 
gravity  upon  the  mass,  which  pulls  the  drop  downwards 
and  elongates  it.  If  he  would  examine  the  problem  in 
its  purity,  he  must  do  as  Plateau  has  done,  detach  the 
liquid  mass  from  the  action  of  gravity  ;  he  would  then 
find  the  shape  to  be  a  perfect  sphere.  Natural  processes 
come  to  us  in  a  mixed  manner,  and  to  the  uninstructed 
mind  are  a  mass  of  unintelligible  confusion.  Suppose 
half-a-dozen  of  the  best  musical  performers  to  be  placed 
in  the  same  room,  each  playing  his  own  instrument  to 
perfection,  but  no  two  playing  the  same  tune  ;  though 
each  individual  instrument  might  be  a  source  of  perfect 
music,  still  the  mixture  of  all  would  produce  mere  noise. 

*  From  a  discourse  delivered  in  the  Royal  Institution  of  Great 
Britain,  June  6,  1856. 


SLATES.  305 

Thiis  it  is  with  the  processes  of  nature,  where  mecha- 
nical and  molecular  laws  intermingle  and  create  ap- 
parent confusion.  Their  mixture  constitutes  what  may 
be  called  the  noise  of  natural  laws,  and  it  is  the  vocation 
of  the  man  of  science  to  resolve  this  noise  into  its  com- 
ponents, and  thus  to  detect  the  underlying  music. 

The  necessity  of  this  detachment  of  one  force  from 
all  other  forces  is  nowhere  more  strikingly  exhibited 
than  in  the  phenomena  of  crystallisation.  Here,  for 
example,  is  a  solution  of  common  sulphate  of  soda  or 
Glauber  salt.  Looking  into  it  mentally,  we  see  the 
molecules  of  that  liquid,  like  disciplined  squadrons 
under  a  governing  eye,  arranging  themselves  into  bat- 
talions, gathering  round  distinct  centres,  and  forming 
themselves  into  solid  masses,  which  after  a  time  assume 
the  visible  shape  of  the  crystal  now  held  in  my  hand. 
I  may,  like  an  ignorant  meddler  wishing  to  hasten 
matters,  introduce  confusion  into  this  order.  This  may 
be  done  by  plunging  a  glass  rod  into  the  vessel ;  the 
consequent  action  is  not  the  pure  expression  of  the  crys- 
talline forces ;  the  molecules  rush  together  with  the 
confusion  of  an  unorganised  mob,  and  not  with  the  steady 
accuracy  of  a  disciplined  host.  In  this  mass  of  bismuth 
also  we  have  an  example  of  confused  crystallisation ;  but 
in  the  crucible  behind  me  a  slower  process  is  going  on: 
here  there  is  an  architect  at  work  '  who  makes  no  chips, 
no  din,'  and  who  is  now  building  the  particles  into 
crystals,  similar  in  shape  and  structure  to  those  beauti- 
ful masses  which  we  see  upon  the  table.  By  permitting 
alum  to  crystallise  in  this  slow  way,  we  obtain  these 
perfect  octahedrons ;  by  allowing  carbonate  of  lime  to 
crystallise,  nature  produces  these  beautiful  rhomboids ; 
when  silica  crystallises,  we  have  formed  these  hexagonal 
prisms  capped  at  the  ends  by  pyramids ;  by  allowing 
saltpetre  to  crystallise  we  have  these  prismatic  masses, 


306  FRAGMENTS  OF  SCIENCE. 

and  when  carbon  crystallises,  we  have  the  diamond.  Ff 
we  wish  to  obtain  a  perfect  crystal  we  must  allow  the 
molecular  forces  free  play  ;  if  the  crystallising  mass  be 
permitted  to  rest  upon  a  surface  it  will  be  flattened, 
and  to  prevent  this  a  small  crystal  must  be  so  suspended 
as  to  be  surrounded  on  all  sides  by  the  liquid,  or,  if  it 
rest  upon  the  surface,  it  must  be  turned  daily  so  as  to 
present  all  its  faces  in  succession  to  the  working  builder. 

In  building  up  crystals  these  little  atomic  bricks 
often  arrange  tliemselves  into  layers  which  are  perfectly 
parallel  to  each  other,  and  which  can  be  separated  by 
mechanical  means ;  this  is  called  the  cleavage  of  the 
crystal.  The  crystal  of  sugar  I  hold  in  my  hand  has 
thus  far  escaped  the  solvent  and  abrading  forces  which 
sooner  or  later  determine  the  fate  of  sugar-candy.  I 
readily  discover  that  it  cleaves  with  peculiar  facility  in 
one  direction.  Again  I  lay  my  knife  upon  this  piece  of 
rocksalt,  and  with  a  blow  cleave  it  in  one  direction. 
Laying  the  knife  at  right  angles  to  its  formfer  position, 
the  crystal  cleaves  again  ;  and  finally  placing  the  knife 
at  right  angles  to  the  two  former  positions,  we  find  a 
third  cleavage.  Eocksalt  cleaves  in  three  directions, 
a,nd  the  resulting  solid  is  this  perfect  cube,  which  may 
be  broken  up  into  any  number  of  smaller  cubes.  Ice- 
land spar  also  cleaves  in  three  directions,  not  at  right 
angles,  but  oblique  to  each  other,  the  resulting  solid 
being  a  rhomboid.  In  each  of  these  cases  tlie  mass 
cleaves  with  equal  facility  in  all  three  directions.  For 
the  sake  of  completeness  I  may  say  that  many  crystals 
cleave  with  unequal  facility  in  different  directions  : 
heavy  spar  presents  an  example  of  this  kind  of  cleavage. 

Tm-n  we  now  to  the  consideration  of  some  other 
phenomena  to  which  the  term  cleavage  may  be  applied. 
Beech,  deal,  and  other  woods  cleave  with  facility  along 
the  fibre,  and  this  cleavage  is  most  perfect  when  the 


SLATES.  307 

edge  of  the  axe  is  laid  across  the  ring's  which  mark  the 
growth  of  the  tree.  If  you  look  at  this  bundle  of  hay 
severed  from  a  rick,  you  will  see  a  sort  of  cleavage  in  it 
also ;  the  stalks  lie  in  horizontal  planes,  and  only  a  small 
force  is  required  to  separate  them  laterally.  But  we 
cannot  regard  the  cleavage  of  the  tree  as  the  same  in 
character  as  that  of  the  hayrick.  In  the  one  case  it  is 
the  molecules  arranging  themselves  according  to  organic 
laws  which  produce  a  cleavable  structure,  in  the  other 
case  the  easy  separation  in  one  direction  is  due  to  the 
mechanical  arrangement  of  the  coarse  sensible  stalks 
of  hay. 

This  sandstone  rock  was  once  a  powder  held  in 
mechanical  suspension  by  water.  The  powder  was  com- 
posed of  two  distinct  parts,  fine  grains  of  sand  and 
small  plates  of  mica.  Imagine  a  wide  strand  covered 
by  a  tide,  or  an  estuary  with  water  which  holds  such 
powder  in  suspension :  how  will  it  sink  ?  The  rounded 
grains  of  sand  will  reach  the  bottom  first,  because  they 
encounter  least  resistance,  the  mica  afterwards,  and 
when  the  tide  recedes  we  have  the  little  plates  sliining 
like  spangles  upon  the  surface  of  the  sand.  Each 
successive  tide  brings  its  charge  of  mixed  powder, 
deposits  its  duplex  layer  day  after  day,  and  finally 
masses  of  immense  thickness  are  piled  up,  which  by 
preserving  the  alternations  of  sand  and  mica  tell  the 
tale  of  their  formation.  Take  the  sand  and  mica,  mix 
them  together  in  water,  and  allow  them  to  subside ; 
they  will  arrange  themselves  in  the  manner  indicated, 
and  by  repeating  the  process  'you  can  actually  build 
up  a  mass  which  shall  be  the  exact  counterpart  of  that 
presented  by  nature.  Now  this  structure  cleaves  with 
readiness  along  the  planes  in  which  the  particles  of 
mica  are  strewn.  Specimens  of  such  a  rock  sent  to  me 
from  Halifax,  and  other  masses  from  the  quarries  of 


308  FRAGMENTS  OF  SCIENCE. 

Over  Darwen  in  Lancashire,  are  here  before  you.  "With 
a  hammer  and  chisel  I  can  cleave  them  into  flags ; 
indeed  these  flags  are  employed  for  roofing  purposes  in 
the  districts  from  which  the  specimens  have  come,  and 
receive  the  name  of  slatestone.'  But  you  will  discern 
without  a  word  from  me,  that  this  cleavage  is  not  a 
crystalline  cleavage  any  more  than  that  of  a  hayrick  is. 
It  is  molar,  not  molecular. 

This,  so  far  as  I  am  aware  of,  has  never  been 
imagined,  and  it  has  been  agreed  among  geologists  not 
to  call  such  splitting  as  this  cleavage  at  all,  but  to 
restrict  the  term  to  a  phenomenon  of  a  totally  different 
character. 

Those  who  have  visited  the  slate  quarries  of  Cumber- 
land and  North  Wales  will  have  witnessed  the  pheno- 
menon to  which  I  refer.  We  have  long  drawn  our 
supply  of  roofing-slates  from  such  quarries;  school-boys 
ciphered  on  these  slates,  they  were  used  for  tombstones 
in  churchyards,  and  for  billiard-tables  in  the  metropolis; 
but  not  until  a  comparatively  late  period  did  men  begin 
to  enquire  how  their  wonderful  structure  was  produced. 
What  is  the  agency  which  enables  us  to  split  Honister 
Crag,  or  the  cliffs  of  Snowdon,  into  laminae  from  crown 
to  base?  This  question  is  at  the  present  moment  one 
of  the  great  difficulties  of  geologists,  and  occupies  their 
attention  perhaps  more  than  any  other.  You  may 
wonder  at  this.  Looking  into  the  quarry  of  Penrhyn, 
you  may  be  disposed  to  offer  the  explanation  I  heard 
given  two  years  ago.  'These  planes  of  cleavage,'  said  a 
friend  who  stood  beside  me  on  the  quarry's  edge,  '  are 
the  plane?  of  stratification  which  have  been  lifted  by 
some  convulsion  into  an  almost  vertical  position.'  But 
this  was  a  mistake,  and  indeed  here  lies  the  grand 
difficulty  of  the  problem.  The  planes  of  cleavage  stand 
iu  most  cases  at  a  high  angle  to  the  bedding.     Thanks 


SLATES.  309 

to  Sir  Roderick  Murchison,  I  am  able  to  place  the  proof 
of  this  before  you.  Here  is  a  specimen  of  slate  in  which 
both  the  planes  of  cleavage  and  of  bedding  are  distinctly 
marked,  one  of  them  making  a  large  angle  with  the 
other.  This  is  common.  The  cleavage  of  slates  then 
is  not  a  question  of  stratification ;  what  then  is  its 
cause? 

In  an  able  and  elaborate  essay  published  in  1835, 
Prof.  Sedgwick  proposed  the  theory  that  cleavage  is  due 
to  the  action  of  crystalline  or  polar  forces  subsequent  to 
the  consolidation  of  the  rock.  '  We  may  affirm,'  he  says, 
*that  no  retreat  of  the  parts,  no  contraction  of  dimen- 
sions in  passing  to  a  solid  state,  can  explain  such 
phenomena.  They  appear  to  me  only  resolvable  on  the 
supposition  that  crystalline  or  polar  forces  acted  upon 
the  whole  mass  simultaneously  in  one  direction  and  with 
adequate  force.'  And  again,  in  another  place ;  '  Crys- 
talline forces  have  re-arranged  whole  mountain  masses, 
producing  a  beautiful  crystalline  cleavage,  passing  alike 
through  all  the  strata.'^  The  utterance  of  such  a  man 
struck  deep,  as  it  ought  to  do,  into  the  minds  of 
geologists,  and  at  the  present  day  there  are  few  who  do 
not  entertain  this  view  either  in  whole  or  in  part.*  The 
boldness  of  the  theory,  indeed,  has,  in  some  cases,  caused 
speculation  to  run  riot,  and  we  have  books  published  on 

•  Tramaotions  of  the  Geological  Society ^  ser.  ii.  vol.  iii.  p.  477. 

•  In  a  letter  to  Sir  Charles  Lyell,  dated  from  the  Cape  of  Gk)od 
Hope  February  20,  1836,  Sir  John  Herschel  writes  as  follows: — *If 
rocks  have  been  so  heated  as  to  allow  of  a  commencement  of  crys- 
tallisation, that  is  to  say,  if  they  have  been  heated  to  a  point  at 
which  the  particles  can  begin  to  move  amongst  themselves,  or  at 
least  OB  their  own  axes,  some  general  law  must  then  determine  the 
position  in  which  these  particles  will  rest  on  cooling.  Probably 
that  position  will  have  some  relation  to  the  direction  in  which  the 
heat  escapes.  Now  when  all  or  a  majority  of  particles  of  the  same 
nature  have  a  general  tendency  to  one  position,  that  must  of  course 
determine  a  cleavage  plane.' 


310  FKAaMENTS  OF  SCIENCE. 

the  action  of  polar  forces  and  geologic  magnetism,  whick 
rather  astonish  those  who  know  something  about  the 
subject.  According  to  this  theory  whole  districts  of 
North  Wales  and  Cumberland,  mountains  included,  are 
neither  more  nor  less  than  the  parts  of  a  gigantic  crystal. 
These  masses  of  slate  were  originally  fine  mud,  composed 
of  the  broken  and  abraded  particles  of  older  rocks. 
They  contain  silica,  alumina,  potash,  soda,  and  mica 
mixed  mechanically  together.  Tu  the  course  of  ages  the 
mixture  became  consolidated,  and  the  theory  before  us 
assumes  that  a  process  of  crystallisation  afterwards  re- 
arranged the  particles  and  developed  in  it  a  single  plane 
of  cleavage.  Though  a  bold,  and  I  think  inadmissible, 
stretch  of  analogies,  this  hypothesis  has  done  good 
service.  Right  or  wrong,  a  thoughtfully  uttered  theory 
has  a  dynamic  power  which  operates  against  intellectual 
stagnation ;  and  even  by  provoking  opposition  is  event- 
ually of  service  to  the  cause  of  truth.  It  would,  however, 
have  been  remarkable  if,  among  the  ranks  of  geologists 
themselves,  men  were  not  found  to  seek  an  explanation 
of  slate-cleavage  involving  a  less  hardy  assumption. 

The  first  step  in  an  enquiry  of  this  kind  is  to  seek 
facts.  This  has  been  done,  and  the  labours  of  Daniel 
Sharpe  (the  late  President  of  the  Geological  Society, 
who,  to  the  loss  of  science  and  the  sorrow  of  all  who 
knew  him,  has  so  suddenly  been  taken  away  from  us), 
Mr.  Henry  Clifton  Sorby,  and  others,  have  furnished  us 
with  a  body  of  facts  associated  with  slaty  cleavage,  and 
having  a  most  important  bearing  upon  the  question. 

Fossil  shells  are  found  in  these  slate-rocks.  I  have 
here  several  specimens  of  such  shells  in  the  actual  rock, 
and  occupying  various  positions  in  regard  to  the  cleavage 
planes.  They  are  squeezed,  distorted,  and  crushed  ;  in 
all  cases  the  distortion  leads  to  the  inference  that  the 
rock  which  contains  these  shells  has  been  subjected  to 


SLATES.  311 

enormous  pressure  in  a  direction  at  right  angles  to  the 
planes  of  cleavage.  The  sliells  are  all  flattened  and 
spread  out  in  these  planes.  Compare  this  fossil  trilobite 
of  normal  proportions-  with  these  others  which  have 
suffered  distortion.  Some  have  lain  across,  some  along, 
and  some  oblique  to  the  cleavage  of  the  slate  in  which 
they  are  found  ;  but  in  all  cases  the  distortion  is  such 
as  required  for  its  production  a  compressing  force  acting 
at  right  angles  to  the  planes  of  cleavage.  As  the  trilo- 
bites  lay  in  the  mud,  the  jaws  of  a  gigantic  vice  appear 
to  have  closed  upon  them  and  squeezed  them  into  the 
shapes  you  see. 

We  sometimes  find  a  thin  layer  of  coarse  gritty 
material,  between  two  layers  of  finer  rock,  through 
which  and  across  the  gritty  layer  pass  the  planes  of 
lamination.  The  coarse  layer  is  found  bent  by  the 
pressure  into  sinuosities  like  a  contorted  ribbon.  Mr. 
Sorby  has  described  a  striking  case  of  this  kind.  This 
crumpling  can  be  experimentally  imitated;  the  amount 
of  compression  might,  moreover,  be  roughly  estimated 
by  supposing  the  contorted  bed  to  be  stretched  out,  its 
length  measured  and  compared  with  the  shorter  distance 
into  which  it  has  been  squeezed.  We  find  in  this  way 
that  the  yielding  of  the  mass  has  been  considerable. 

Let  me  now  direct  your  attention  to  another  proof" 
of  pressure ;  you  see  the  varying  colours  which  indicate 
the  bedding  on  this  mass  of  slate.  The  dark  portion  is 
gritty,  being  composed  of  comparatively  coarse  par- 
ticles, which,  owing  to  their  size,  shape  and  gravity, 
sink  first  and  constitute  the  bottom  of  each  layer. 
Grradually,  from  bottom  to  top  the  coarseness  diminishes, 
and  near  the  upper  surface  we  have  a  layer  of  exceed- 
ingly fine  grain.  It  is  the  fine  mud  thus  consolidated 
from  which  are  derived  the  German  razor-stones,  so 
much  prized  for  the  sharpening  of  surgical  instruments. 
21 


312  FEAGMENTS  OF  SCIENCE 

Wlien  a  bed  is  thin,  the  fine-grain  slate  is  permitted  to 
rest  upon  a  slab  of  the  coarse  slate  in  contact  with  it ; 
when  the  fine  bed  is  thick,  it  is  cut  into  slices  which  are 
cemented  to  pieces  of  ordinary  slate,  and  thus  rendered 
stronger.  The  mud  thus  deposited  is,  as  might  be 
expected,  often  rolled  up  into  nodular  masses,  carried 
forward,  and  deposited  among  coarser  material  by  the 
rivers  from  which  the  slate-mud  has  subsided.  Here 
are  such  nodules  enclosed  in  sandstone.  Everybody, 
moreover,  who  has  ciphered  upon  a  school-slate  must 
remember  the  whitish-green  spots  which  sometimes 
dotted  the  surface  of  the  slate,  and  over  which  the 
pencil  usually  slid  as  if  the  spots  were  greasy.  Now 
these  spots  are  composed  of  the  finer  mud,  and  they 
could  not,  on  account  of  their  fineness,  bite  the  pencil 
like  the  siu-rounding  gritty  portions  of  the  slate. 
Here  is  a  beautiful  example  of  these  spots  :  you  observe 
them,  on  the  cleavage  surface,  in  broad  round  patches. 
But  turn  the  slate  edgeways  and  the  section  of  each 
nodule  is  seen  to  be  a  sharp  oval  with  its  longer  axis 
parallel  to  the  cleavage.  This  instructive  fact  has 
been  adduced  by  Mr.  Sorby.  I  have  made  excursions 
to  the  quarries  of  Wales  and  Cumberland,  and  to  many 
of  the  slate  yards  of  London,  and  found  the  fact 
general.  Thus  we  elevate  a  common  experience  of  our 
boyhood  into  evidence  of  the  hi^^hest  significance  as 
regards  a  most  important  geological  problem.  From 
the  magnetic  deportment  of  these  slates,  I  was  led  to 
infer  that  these  spots  contain  a  less  amount  of  iron 
than  the  surrounding  dark  slate.  An  analysis  was 
made  for  me  by  Mr.  Hambly  in  the  laboratory  of  Dr. 
Percy  at  the  School  of  Mines  with  the  following 
result : — 


SLATES.  313 

ANALYSIS  OP  SLATB, 

Ba/rk  Slater  two  analyses, 

1.  Percentage  of  iron  ••••••    5*85 

a.  n  6:13 

Mean  •        •    5*99 

Whitish  Green  Slate, 

1.  Percentage  of  iron  •••.••    3*24 
2«  n  !»••••••    3*12 

Mean  .        ,    3*18 

According  to  these  analyses  the  quantity  of  iron  in  the 
dark  slate  immediately  adjacent  to  the  greenish  spot 
is  nearly  double  the  quantity  contained  in  the  spot 
itself.  This  is  about  the  proportion  which  the  mag- 
netic experiments  suggested. 

Let  me  now  remind  you  that  the  facts  brought 
before  you  are  typical — each  is  the  representative  of 
a  class.  We  have  seen  shells  crushed ;  the  trilo- 
bites  squeezed,  beds  contorted,  nodules  of  greenish 
marl  flattened ;  and  all  these  sources  of  independent 
testimony  point  to  one  and  the  same  conclusion, 
namely,  that  slate-rocks  have  been  subjected  to 
enormous  pressure  in  a  direction  at  right  angles  to  the 
planes  of  cleavage. 

In  reference  to  Mr.  Sorby's  contorted  bed,  I  have 
said  that  by  supposing  it  to  be  stretched  out  and  its 
length  measured,  it  would  give  us  an  idea  of  the 
amount  of  yielding  of  the  mass  above  and  below  the  bed. 
Such  a  measurement,  however,  would  not  give  the 
exact  amount  of  yielding.  I  hold  in  my  hand  a  specimen 
of  slate  with  its  bedding  marked  upon  it ;  the  lower 
portions  of  each  layer  being  composed  of  a  com- 
paratively coarse  gritty  material  something  like  what 
you  may  suppose  the  contorted  bed  to  be  composed  of. 
Now  in  crossing  these  gritty  portions,  the  cleavage  turns, 


514  FRAGMENTS  OF  SCIENCE. 

as  if  tending  to  cross  the  bedding  at  another  angle. 
When  the  pressure  began  to  act,  the  intermediate  bed, 
which  is  not  entirely  unyielding,  suffered  longitudinal 
pressure;  as  it  bent,  the  pressure  became  gradually 
more  transverse,  and  the  direction  of  its  cleavage  is 
exactly  such  as  you  would  infer  from  an  action  of  this 
kind — it  is  neither  quite  across  the  bed,  nor  yet  in  the 
same  direction  as  the  cleavage  of  the  slate  above  and 
below  it,  but  intermediate  between  both.  Supposing 
the  cleavage  to  be  at  right  angles  to  the  pressure,  this 
is  the  direction  which  it  ought  to  take  across  these 
more  unyielding  strata. 

Thus  we  have  established  the  concurrence  of  the 
phenomena  of  cleavage  and  pressure — that  they  accom- 
pany each  other ;  but  the  question  still  remains,  Is  the 
pressure  sufficient  to  account  for  the  cleavage  ?  A 
single  geologist,  as  far  as  I  am  aware,  answers  boldly  in 
the  affirmative.  This  geologist  is  Sorby,  who  has  attacked 
the  question  in  the  true  spirit  of  a  physical  investigator. 
Call  to  mind  the  cleavage  of  the  flags  of  Halifax  and 
Over  Darwen,  which  is  caused  by  the  interposition  of 
layers  of  mica  between  the  gritty  strata.  Mr.  Sorby 
finds  plates  of  mica  to  be  also  a  constituent  of  slate- 
rock.  He  asks  himself,  what  will  be  the  effect  of 
pressure  upon  a  mass  containing  such  plates  confusedly 
mixed  up  in  it  ?  It  will  be,  he  argues,  and  he  argues 
rightly,  to  place  the  plates  with  their  flat  surfaces 
more  or  less  perpendicular  to  the  direction  in  which 
the  pressure  is  exerted.  He  takes  scales  of  the  oxide 
of  iron,  mixes  them  with  a  fine  powder,  and  on  squeez- 
ing the  mass  finds  that  the  tendency  of  the  scales  is  to 
set  themselves  at  right  angles  to  the  line  of  pressure. 
Along  the  planes  of  weakness  produced  by  the  scales 
the  mass  cleaves. 

By  tests  of  a  different  character  from  those  applied 


SLATES.  315 

by  Mr.  Sorby,  it  might  be  sbown  how  true  his  con- 
clusion is — that  the  effect  of  pressure  on  elongated 
particles,  or  plates,  will  be  such  as  he  describes  it. 
But  while  the  scales  must  be  regarded  as  a  true  cause, 
I  should  not  ascribe  to  them  a  large  share  in  the  pro- 
duction of  the  cleavage.  I  believe  that  even  if  the 
plates  of  mica  were  wholly  absent,  the  cleavage  of 
slate-rocks  would  be  much  the  same  as  it  is  at  present. 

Here  is  a  mass  of  pure  white  wax :  it  contains  no 
mica  particles,  no  scales  of  iron,  or  anything  analogous 
to  them.  Here  is  the  selfsame  substance  submitted  to 
pressure.  I  would  invite  the  attention  of  the  eminent 
geologists  now  before  me  to  the  structure  of  this  wax. 
No  slate  ever  exhibited  so  clean  a  cleavage ;  it  splits 
into  laminae  of  surpassing  tenuity,  and  proves  at  a 
single  stroke  that  pressure  is  sufHcient  to  produce 
cleavage,  and  that  this  cleavage  is  independent  of  inter- 
mixed plates  or  scales.  I  have  purposely  mixed  this 
wax  with  elongated  particles,  and  am  unable  to  say  at 
the  present  moment  that  the  cleavage  is  sensibly 
affected  by  their  presence — if  anything,  I  should  say 
they  rather  impair  its  fineness  and  clearness  than  pro- 
mote it. 

The  finer  the  slate  is  the  more  perfect  will  be  the 
resemblance  of  its  cleavage  to  that  of  the  wax.  Com- 
pare the  surface  of  the  wax  with  the  surface  of  this 
slate  from  Borrodale  in  Cumberland.  You  have  pre-- 
cisely  the  same  features  in  both :  you  see  flakes  clinging 
to  the  surfaces  of  each,  which  have  been  partially  torn 
away  in  cleaving.  Let  any  close  observer  compare 
these  two  effects,  he  will,  I  am  persuaded,  be  led  to 
the  conclusion  that  they  are  the  product  of  a  common 
cause.* 

*  I  have  usually  softened  the  wax  by  warming  it,  kneaded  it 
with  the  lingers,  and  pressed  it  between  thick  plates  of  glass  pre- 


316  FRAGMENTS  OF  SCIENCE. 

But  you  will  ask  me  how,  according  to  my  view, 
does  pressure  produce  this  remarkable  result?  This 
may  be  stated  in  a  very  few  words. 

There  is  no  such  thing  in  nature  as  a  body  of 
perfectly  homogeneous  structure.  I  break  this  clay 
which  seems  so  uniform,  and  find  that  the  fracture 
presents  to  my  eyes  innumerable  surfaces  along  which 
it  has  given  way,  audit  has  yielded  along  those  surfaces 
because  in  them  the  cohesion  of  the  mass  is  less  than 
elsewhere.  I  break  this  marble,  and  even  this  wax, 
and  observe  the  same  result ;  look  at  the  mud  at  the 
bottom  of  a  dried  pond ;  look  at  some  of  the  ungravelled 
walks  in  Kensington  Gardens  on  drying  after  rain, — 
they  are  cracked  and  split,  and  other  circumstances 
being  equal,  they  crack  and  split  where  the  cohesion  is 
a  minimum.  Take  then  a  mass  of  partially  consolidated 
mud.  Such  a  mass  is  divided  and  subdivided  by  interior 
surfaces  along  which  the  cohesion  is  comparatively 
small.  Penetrate  the  mass  in  idea,  and  you  will  see  it 
composed  of  numberless  irregular  polyhedra  bounded 
by  surfaces  of  weak  cohesion.  Imagine  such  a  mass 
subjected  to  pressure, — it  yields  and  spreads  out  in  the 
direction  of  least  resistance ;  ^  the  little  polyhedra  be- 
come converted  into  laminae,  separated  from  each  other 
by  surfaces  of  weak  cohesion,  and  the  infallible  result 

viously  wetted.  At  the  ordinary  snnimer  temperature  the  pressed 
wax  is  soft,  and  tears  rather  than  cleaves  ;  on  this  account  I  cool 
my  compressed  specimens  in  a  mixture  of  pounded  ice  and  salt, 
and  when  thus  cooled  they  split  cleanly. 

•  It  is  scarcely  necessary  to  say  that  if  the  mass  were  squeezed 
equally  in  all  directions  no  laminated  structure  could  be  produced; 
it  must  have  room  to  yield  in  a  lateral  direction.  Mr.  Warren  De 
la  Rue  informs  me  that  he  once  wished  to  obtain  white-lead  in  a 
fine  granular  state,  and  to  accomplish  this  he  first  compressed  it. 
The  mould  was  conical,  and  permitted  the  lead  to  spread  out  a 
little  laterally.  The  lamination  was  as  perfect  as  that  of  slate,  and 
It  quite  defeated  him  in  his  effort  to  obtain  a  granular  powder. 


SLATES.  317 

will  be  a  tendency  to  cleave  at  right  angles  to  the  line 
of  pressure. 

*  Further,  a  mass  of  dried  mud  is  full  of  cavities  and 
fissures.  If  you  break  dried  pipe-clay  you  see  them  in 
great  numbers,  and  there  are  multitudes  of  them  so 
small  that  you  cannot  see  them.  A  flattening  of  these 
cavities  must  take  place  in  squeezed  mud,  and  this 
must  to  some  extent  facilitate  the  cleavage  of  the  mass 
in  the  direction  indicated. 

Although  the  time  at  my  disposal  has  not  permitted 
me  duly  to  develope  these  thoughts,  yet  for  the  last 
twelve  months  the  subject  has  presented  itself  to  me 
almost  daily  under  one  aspect  or  another.  I  have 
never  eaten  a  biscuit  during  this  period  without  re- 
marking the  cleavage  developed  by  the  rolling-pin. 
You  have  only  to  break  a  biscuit  across,  and  to  look  at 
the  fracture,  to  see  the  laminated  structure.  We  have 
here  the  means  of  pushing  the  analogy  further.  I  in- 
vite you  to  compare  the  structure  of  this  slate,  which 
was  subjected  to  a  high  temperature  during  the  confla- 
gration of  Mr.  Scott  Eussell's  premises,  with  that  of  a 
biscuit.  Air  or  vapour  within  the  slate  has  caused  it 
to  swell,  and  the  mechanical  structure  it  reveals  is  pre- 
cisely that  of  a  biscuit.  During  these  enquiries  I  have 
received  much  instruction  in  the  manufacture  of  puff- 
paste.  Here  is  some  such  paste  baked  under  my  own 
superintendence.  The  cleavage  of  our  hills  is  acci- 
dental cleavage,  but  this  is  cleavage  with  intention. 
The  volition  of  the  pastrycook  has  entered  into  its 
formation.  It  has  been  his  aim  to  preserve  a  series  of 
surfaces  of  structural  weakness,  along  which  the  dough 
divides  into  layers.  Puff-paste  in  preparation  must 
not  be  handled  too  much ;  it  ought,  moreover,  to  be 
rolled  on  a  cold  slab,  to  prevent  the  butter  from  melt- 


818  FRAGMENTS  OF  SCIENCE. 

ing,  and  diffusing  itself,  thus  rendering  the  paste  more 
homogeneous  and  less  liable  to  split.  Puff-paste  is, 
then,  simply  an  exaggerated  case  of  slaty  cleavage.    ' 

The  principle  here  enunciated  is  so  simple  as  to 
be  almost  trivial ;  nevertheless,  it  embraces  not  only 
the  cases  mentioned,  but,  if  time  permitted,  it  might 
be  shown  you  that  the  principle  has  a  much  wider 
range  of  application.  When  iron  is  taken  from  the 
puddling  furnace  it  is  more  or  less  spongy,  an  aggre- 
gate n  fact  of  small  nodules :  it  is  at  a  welding  heat, 
and  at  this  temperature  is  submitted  to  the  process  of 
rolling.  Bright  smooth  bars  are  the  result.  But  not- 
withstanding the  high  heat  the  nodules  do  not  perfectly 
blend  together.  The  process  of  rolling  draws  them  into 
fibres.  Here  is  a  mass  acted  upon  by  dilute  sulphuric 
acid,  which  exhibits  in  a  striking  manner  this  fibrous 
structure.  The  experiment  was  made  by  my  friend  Dr. 
Percy,  without  any  reference  to  the  question  of  cleavage. 

Break  a  piece  of  ordinary  iron  and  you  have  a  granu- 
lar fracture ;  beat  the  iron,  you  elongate  these  granules, 
and  finally  render  the  mass  fibrous.  Here  are  pieces  of 
rails  along  which  the  wheels  of  locomotives  have  slid- 
den ;  the  granules  have  yielded  and  become  plates. 
They  exfoliate  or  come  ofi^  in  leaves ;  all  these  effects 
belong,  I  believe,  to  the  great  class  of  phenomena 
of  which  slaty  cleavage  forms  the  most  prominent 
example.* 

We  have  now  reached  the  termination  of  our  task. 
You  have  witnessed  the  phenomena  of  crystallisation, 
and  have  had  placed  before  you  the  facts  which  are  found 
associated  with  the  cleavage  of  slate  rocks.  Such  facts, 
as  expressed  by  Helmholtz,  are  so  many  telescopes 
to  our  spiritual  vision,  by  which  we  can  see  backward 

>  For  some  further  observations  on  this  subject  by  Mr.  Sorby 
and  myself,  see  PhiloHophical  Magazine  for  August,  1856. 


SLATES.  319 

through  the  night  of  antiquity,  and  discern  the  forces 
which  have  been  in  operation  upon  the  earth's  surface 

Ere  the  lion  roared, 
Or  the  eagle  soared. 

From  evidence  of  the  most  independent  and 
trustworthy  character,  we  come  to  the  conclusion 
that  these  slaty  masses  have  been  subjected  to  enormous 
pressure,  and  by  the  sure  method  of  experiment  we 
have  shown — and  this  is  the  only  really  new  point 
which  has  been  brought  before  you — how  the  pressure 
is  sufficient  to  produce  the  cleavage.  Expanding  our 
field  of  view,  we  find  the  self-same  law,  whose  footsteps  we 
trace  amid  the  crags  of  Wales  and  Cumberland,  extend- 
ing into  the  domain  of  the  pastrycook  and  ironfounder ; 
nay,  a  wheel  cannot  roll  over  the  half-dried  mud  of  our 
streets  without  revealing  to  us  more  or  less  of  the  features 
of  this  law.  Let  me  say,  in  conclusion,  that  the  spirit 
in  which  this  problem  has  been  attacked  by  geologists, 
indicates  the  dawning  of  a  new  day  for  their  science. 
The  great  intellects  who  have  laboured  at  geology,  and 
who  have  raised  it  to  its  present  pitch  of  grandeur,  were 
compelled  to  deal  with  the  subject  in  mass  ;  they  had 
no  time  to  look  after  details.  Rut  the  desire  for  more 
exact  knowledge  is  increasing;  facts  are  flowing  in 
which,  while  they  leave  untouched  the  intrinsic  wonders 
of  geology,  are  gradually  supplanting  by  solid  truths  the 
uncertain  speculations  which  beset  the  subject  in  its  in- 
fancy. Greologists  now  aim  to  imitate,  as  far  as  possible, 
the  conditions  of  nature,  and  to  produce  her  results ; 
they  are  approaching  more  and  more  to  the  domain  of 
physics,  and  I  trust  the  day  will  soon  come  when  we 
shall  interlace  our  friendly  arms  across  the  common 
boundary  of  oiu:  sciences,  and  pursue  om*  respective  tasks 


320  FRAaMENTS  OF  SCIENCE. 

in  a  spirit  of  mutual  helpfulness,  encouragement  and 
goodwilL 

[I  would  now  lay  more  stress  on  the  lateral  yielding, 
referred  to  in  the  note  at  the  hottom  of  page  316, 
accompanied  as  it  is  by  tangential  sliding,  than  I  was 
prepared  to  do  when  this  lecture  was  given.  This  sliding 
is,  I  think,  the  principal  cause  of  the  planes  of  weak- 
ness, both  in  pressed  wax  and  slate  rock.     J.  T.   1871.] 


XIII. 
ON  PABAMA  QNETIG  AND  DIAMA  GNETIG  FORCES.^ 

THE  notion  of  an  attractive  force,  which  draws  bodies 
towards  the  centre  of  the  earth,  was  entertained  by 
Anaxagoras  and  his  pupils,  by  Democritus,  Pythagoras, 
and  Epicurus ;  and  the  conjectures  of  these  ancients  were 
renewed  by  Gralileo,  Huyghens,  and  others,  who  stated 
that  bodies  attract  each  other  as  a  magnet  attracts  iron. 
Kepler  applied  the  notion  to  bodies  beyond  the  surface 
of  the  earth,  and  affirmed  the  extension  of  this  force  to 
the  most  distant  stars.  Thus  it  would  appear,  that  in 
the  attraction  of  iron  by  a  magnet  originated  the  con- 
ception of  the  force^  of  gravitation.  Nevertheless,  if  we 
look  closely  at  the  matter,  it  will  be  seen  that  the  mag- 
netic force  possesses  characters  strikingly  distinct  from 
those  of  the  force  which  holds  the  universe  together. 
The  theory  of  gravitation  is,  that  every  particle  of 
matter  attracts  every  other  particle ;  in  magnetism  also 
we  have  attraction,  but  we  have  always,  at  the  same 
time,  repulsion,  the  final  effect  being  due  to  the  differ- 
ence of  these  two  forces.  A  body  may  be  intensely 
acted  on  by  a  magnet,  and  still  no  motion  of  translation 
will  follow,  if  the  repulsion  be  equal  to  the  attraction. 
Previous  to  magnetization,  a  dipping  needle,  when  its 
centre  of  gravity  is  supported,  stands  accurately  level ; 
but,  after  magnetization,  one  end  of  it,  in  our  latitude, 

»  Abstract  of  a  discourse  delivered  in  the  Eoyal  Institution, 
Februaxy  1,  1856. 


322  FRAGMENTS  OF  SCIENCE. 

is  pulled  towards  the  north  pole  of  the  earth.  The 
needle,  however,  being  suspended  from  the  arm  of  a  fine 
balance,  its  weight  is  found  unaltered  by  its  magneti- 
zation. In  like  manner,  when  the  needle  is  permitted 
to  float  upon  a  liquid,  and  thus  to  follow  the  attraction  of 
the  north  magnetic  pole  of  the  earth,  there  is  no  motion 
of  the  mass  towards  that  pole.  The  reason  is  known 
to  be,  that  although  the  marked  end  of  the  needle  is 
attracted  by  the  north  pole,  the  unmarked  end  is 
repelled  by  an  equal  force,  the  two  equal  and  opposite 
forces  neutralizing  each  other. 

When  the  pole  of  an  ordinary  magnet  is  brought 
to  act  upon  the  swimming  needle,  the  latter  is  at- 
tracted,— the  reason  being  that  the  attracted  end  of 
the  needle  being  nearer  to  the  pole  of  the  magnet 
than  the  repelled  end,  the  force  of  attraction  is  the 
more  powerful  of  the  two.  In  the  case  of  the  earth, 
its  pole  is  so  distant  that  the  length  of  the  needle  is 
practically  zero.  In  like  manner,  when  a  piece  of  iron 
is  presented  to  a  magnet,  the  nearer  parts  are  attracted, 
while  the  more  distant  parts  are  repelled ;  and  because 
the  attracted  portions  are  nearer  to  the  magnet  than  the 
repelled  ones,  we  have  a  balance  in  favour  of  attraction. 
Here  then  is  the  special  characteristic  of  the  magnetic 
force,  which  distinguishes  it  from  that  of  gravitation. 
The  latter  is  a  simple  un polar  force,  while  the  former 
is  duplex  or  polar.  Were  gravitation  like  magnetism, 
a  stone  would  no  more  fall  to  the  ground  than  a  piece 
of  iron  towards  the \ north  magnetic  pole:  and  thus, 
however  rich  in  consequences  the  supposition  of  Kepler 
and  others  may  have  been,  it  is  clear  that  a  force  like 
that  of  magnetism  would  not  be  able  to  transact  the 
business  of  the  universe. 

The  object  of  this  discourse  is  to  enquire  whether 
the  force  of  diamagnetism,  which  manifests  itself  as  a 


ON  PAEAMAaNETIC   AND   DIAMAGNETIC  FORCES.     323 

repulsion  of  certain  bodies  by  the  poles  of  a  magnet, 
is  to  be  ranged  as  a  polar  force,  beside  that  of  magne- 
tism ;  or  as  an  unpolar  force,  beside  that  of  gravitation. 
When  a  cylinder  of  soft  iron  is  placed  within  a  wire 
helix,  and  surrounded  by  an  electric  current,  the  anti- 
theais  of  its  two  ends,  or,  in  other  words,  its  polar  ex- 
citation, is  at  once  manifested  by  its  action  upon  a 
magnetic  needle ;  and  it  may  be  asked  why  a  cylinder 
of  bismuth  may  not  be  substituted  for  the  cylinder  of 
iron,  and  its  state  similarly  examined.  The  reason  is, 
that  the  excitement  of  the  bismuth  is  so  feeble,  that  it 
would  be  quite  masked  by  that  of  the  helix  in  which  it 
is  enclosed ;  and  the  problem  that  now  meets  us  is,  so 
to  excite  a  diamagnetic  body  that  the  pure  action  of  the 
body  upon  a  magnetic  needle  may  be  observed,  un- 
mixed with  the  action  of  the  body  used  to  excite  the 
diamagnetic. 

How  this  has  been  effected  may  be  illustrated  in  the 
following  manner : — When  through  an  upright  helix 
of  covered  copper  wi/e,  a  voltaic  current  is  sent,  the  top 
of  the  helix  attracts,  while  its  bottom  repels,  the  same 
pole  of  a  magnetic  needle  ;  its  central  point,  on  the  con- 
trary, is  neutral,  and  exhibits  neither  attraction  nor 
repulsion.     Such  a  helix  is  caused  to  stand  between  the 


NC 


JH^ 


jS' 


Fig.  10. 

two  poles  n's'  of  an  astatic  system.^    The  two  magnets 

sn'    and   s'n   are   united   by  a   rigid   cross   piece   at 

'  The  reversal  of  the  poles  of  the  two  magnets,  which  were  of 
the  same  strength,  completely  annulled  the  action  of  the  earth  as  a 
magnet. 


324  FKAGMENTS  OF  SCIENCE. 

their  centres,  and  axe  suspended  from  the  point  a,  so  that 
both  magnets  swing  in  the  same  horizontal  plane.  It 
is  so  arranged  that  the  poles  n'  s'  are  opposite  to  the 
central  or  neutral  point  of  the  helix,  so  that  when  a 
current  is  sent  through  the  latter,  the  magnets,  as 
before  explained,  are  unaffected.  Here  then  we  have 
an  excited  helix  which  itself  has  no  action  upon  the 
magnets,  and  we  are  thus  enabled  to  examine  the 
action  of  a  body  placed  within  the  helix  and  excited 
by  it,  undisturbed  by  the  influence  of  the  latter.  The 
helix  being  12  inchej'  ^lig'h,  a  cylinder  of  soft  iron  6 
inches  long,  suspended  from  a  string  and  passing  over 
a  pulley,  can  be  raised  or  lowered  within  the  helix. 
When  it  is  so  far  sunk  that  its  lower  end  rests  upon 
the  table,  the  upper  end  finds  itself  between  the  poles 
n'  s'  of  the  astatic  system.  The  iron  cylinder  is  thus 
converted  into  a  strong  magnet,  attracting  one  of  the 
poles,  and  repelling  the  other,  and  consequently  deflect- 
ing the  entire  astatic  system.  When  the  cylinder  is 
raised  so  that  the  upper  end  is  at  the  level  of  the  top 
of  the  helix,  its  lower  end  comes  between  the  poles 
n's';  and  a  deflection  opposed  in  direction  to  the 
former  one  is  the  immediate  consequence.  To  render 
these  deflections  more  easily  visible,  a  mirror  m  is 
attached  to  the  system  of  magnets ;  a  beam  of  light 
thrown  upon  the  mirror  being  reflected  and  projected 
as  a  bright  disk  against  the  wall.  The  distance  of 
this  image  from  the  mirror  being  considerable,  and  its 
angular  motion  double  that  of  the  latter,  a  very  slight 
motion  of  the  magnet  is  sufficient  to  produce  a  dis- 
placement of  the  image  through  several  yards. 

This  then  is  the  principle  of  the  beautiful  apparatus* 

'  Devised  hj  Prof.  W.  "Weber,  and  constructed  by  M.  Leyser, 
of  Leipzig. 


ON  PARAMAaNETIC  AND  DIAMAGNETIC  FORCES.     325 

by  which  the  investigation  was  conducted.  It  is  mani- 
fest that  if  a  second  helix  be  placed  between  the  poles 
s  N  with  a  cylinder  within  it,  the  action  upon  the 
astatic  magnet  may  be  exalted.  This  was  the  arrange- 
ment made  use  of  in  the  actual  enquiry.  Thus  to 
intensify  the  feeble  action,  which  it  is  here  our  object 
to  seek,  we  have  in  the  first  place  neutralized  the  action 
of  the  earth  upon  the  magnets,  by  placing  them  asta- 
tically.  Secondly,  by  making  use  of  two  cylinders,  and 
permitting  them  to  act  simultaneously  on  the  four  poles 
of  the  magnets,  we  have  rendered  the  deflecting  force 
four  times  what  it  would  be,  if  only  a  single  pole 
were  used.  Finally,  the  whole  apparatus  was  en- 
closed in  a  suitable  case  which  protected  the  magnets 
from  air-currents,  and  the  deflections  were  read  off 
through  a  glass  plate  in  the  case,  by  means  of  a  tele- 
scope and  scale  placed  at  a  considerable  distance  from 
the  instrument. 

A  pair  of  bismuth  ^cylinders  was  first  examined. 
Sending  a  current  through  the  helices,  and  observing 
that  the  magnets  swung  perfectly  free,  it  was  first  ar- 
ranged that  the  bismuth  cylinders  within  the  helices 
had  their  central  or  neutral  points  opposite  to  the  poles 
of  the  magnets.  All  being  at  rest  the  number  on  the 
scale  marked  by  the  cross  wire  of  the  telescope  was  572. 
The  cylinders  were  then  moved,  one  up  the  other  down, 
so  that  two  of  their  ends  were  brought  to  bear  simul- 
taneously upon  the  magnetic  poles  :  the  magnet  moved 
promptly,  and  after  some  oscillations^  came  to  rest  at 
the  number  612 ;  thus  moving  from  a  smaller  to  a 
larger  number.  The  other  two  ends  of  the  bars  were 
next  brought  to  bear  upon  the  magnet :  a  prompt  deflec- 
tion was  the  consequence,  and  the  final  position  of 
•  To  lessen  these  a  copper  damper  was  made  use  of. 


326  FRAGMENTS    OF  SCIENCE. 

equilibrium  was  526  :  the  movement  being  from  a  larger 
to  a  smaller  number.  We  thus  observe  a  manifest  polar 
action  of  the  bismuth  cylinders  upon  the  magnet ;  one 
pair  of  ends  deflecting  it  in  one  direction,  and  the  other 
pair  deflecting  it  in  the  opposite  direction. 

Substituting  for  the  cylinders  of  bismuth  thin 
cylinders  of  iron,  of  magnetic  slate,  of  sulphate  of  iron, 
carbonate  of  iron,  protochloride  of  iron,  red  ferrocyanide 
of  potassium,  and  other  magnetic  bodies,  it  was  found 
that  when  the  position  of  the  magnetic  cylinders  was 
the  same  as  that  of  the  cylinders  of  bismuth,  the  deflec- 
tion produced  by  the  former  was  always  opposed  in 
direction  to  that  produced  by  the  latter ;  and  hence  the 
disposition  of  the  force  in  the  diamagnetic  body  must 
have  been  precisely  antithetical  to  its  disposition  in  the 
magnetic  ones. 

But  it  will  be  urged,  and  indeed  has  been  urged 
against  this  inference,  that  the  deflection  produced  by 
the  bismuth  cylinders  may  be  due  to  induced  currents 
excited  in  the  metal  by  its  motion  within  the  helices. 
In  reply  to  this  objection,  it  may  be  stated,  in  the 
first  place,  that  the  deflection  is  permanent,  and  can- 
not therefore  be  due  to  induced  currents,  which  are  only 
of  momentary  duration.  It  has  also  been  luged  that 
such  experiments  ought  to  be  made  with  other  metals, 
and  with  better  conductors  than  bismuth ;  for  if  due 
to  currents  of  induction,  the  better  the  conductor  the 
more  exalted  will  be  the  efifect.  This  requirement  was 
complied  with. 

Cylinders  of  antimony  were  substituted  for  those  of 
bismuth.  This  metal  is  a  better  conductor  of  elec- 
tricity, but  less  strongly  diamagnetic  than  bismuth. 
If  therefore  the  action  referred  to  be  due  to  induced 
currents  we  ought  to  have  it  greater  in  the  case  of  anti- 
mony than  with  bismuth ;  but  if  it  springs  from  a  true 


ON  PAEAMAGNETIC  AND^  DIAMAONETIC  FORCES.     327 

diamagnetic  polarity,  the  action  of  the  bismuth  ought 
to  exceed  that  of  the  antimony.  Experiment  proves 
this  to  be  the  case.  Hence  the  deflection  produced  by 
these  metals  is  due  to  their  diamagnetic,  and  not  to 
their  conductive  capacity.  Copper  cylinders  were  next 
examined  :  here  we  have  a  metal  which  conducts  elec- 
tricity fifty  times  better  than  bismuth,  but  its  diamag- 
netic power  is  nearly  null;  if  the  effects  be  due  to 
induced  currents  we  ought  to  have  them  here  in  an 
enormously  exaggerated  degree,  but  no  sensible  deflec- 
tion was  produced  by  the  two  cylinders  of  copper. 

It  has  also  been  proposed  by  the  opponents  of  dia- 
magnetic polarity  to  coat  fragments  of  bismuth  with 
some  insulating  substance,  so  as  to  render  the  formation 
of  induced  currents  impossible,  and  to  test  the  question 
with  cylinders  of  these  fragments.  This  requirement 
was  also  fulfilled.  It  is  only  necessary  to  reduce  the 
bismuth  to  powder  and  expose  it  for  a  short  time  to  the 
air  to  cause  the  particles  to  become  so  far  oxidised  as 
to  render  them  perfectly  insulating.  The  -insulating 
power  of  the  powder  was  exhibited  experimentally; 
nevertheless,  this  powder,  enclosed  in  glass  tubes,  ex- 
hibited an  action  scarcely  less  powerful  than  that  of  the 
massive  bismuth  cylinders. 

Bat  the  most  rigid  proof,  a  proof  admitted  to  be 
conclusive  by  those  who  have  denied  the  antithesis  of 
magnetism  and  diamagnetism,  remains  to  be  stated. 
Prisms  of  the  same  heavy  glass  as  that  with  which  the 
diamagnetic  force  was  discovered,  were  substituted  for 
the  metallic  cylinders,  and  their  action  upon  the  mag- 
net was  proved  to  be  precisely  the  same  in  kind  as  that 
of  the  cylinders  of  bismuth.  The  enquiry  was  also 
extended  to  other  insulators :  to  phosphorus,  sulphur, 
nitre,  calcareous  spar,  statuary  marble,  with  the  same 
invariable  result :  each  of  these  substances  was  proved 
22. 


328  FRAGMENTS  OF  SCIENCE. 

to  be  polar,  the  disposition  of  the  force  being  the  same  as 
that  of  bismuth  and  the  reverse  of  that  of  iron.  When 
a  bar  of  iron  is  set  erect,  its  lower  end  is  known  to  be 
a  north  pole,  and  its  upper  end  a  south  pole,  in  virtue 
of  the  earth's  induction.  A  marble  statue,  on  the  con- 
trary, has  its  feet  a  south  pole,  and  its  head  a  north 
pole,  and  there  is  no  doubt  that  the  same  remark  applies 
to  its  living  archetype ;  each  man  walking  over  the 
earth's  surface  is  a  true  diamagnet,  with  its  poles  the 
reverse  of  those  of  a  mass  of  magnetic  matter  of  the 
same  shape  and  position. 

An  experiment  of  practical  value,  as  affording  a 
ready  estimate  of  the  different  conductive  powers  of 
two  metals  for  electricity,  was  exhibited  in  the  lecture, 
for  the  purpose  of  proving  experimentally  some  of  the 
statements  made  in  reference  to  this  subject.  A  cube 
of  bismuth  was  suspended  by  a  twisted  string  between 
the  two  poles  of  an  electro-magnet.  The  cube  was 
attached  by  a  short  copper  wire  to  a  little  square 
pyramid,  the  base  of  which  was  horizontal,  and  its  sides 
formed  of  four  small  triangular  pieces  of  looking-glass. 
A  beam  of  light  was  suffered  to  fall  upon  this  reflector, 
and  as  the  reflector  followed  the  motion  of  the  cube 
the  images  cast  from  its  sides  followed  each  other  in 
succession,  each  describing  a  circle  about  thirty  feet 
in  diameter.  As  the  velocity  of  rotation  augTuented, 
these  images  blended  into  a  continuous  ring  of  light. 
At  a  particular  instant  the  electro- magnet  was  excited, 
currents  were  evolved  in  the  rotating  cube,  and  the 
strength  of  these  currents,  which  increases  with  the 
conductivity  of  the  cube  for  electricity,  was  practically 
estimated  by  the  time  required  to  bring  the  cube  and 
its  associated  mirrors  to  a  state  of  rest.  With  bismuth 
this  time  amounted  to  a  score  of  seconds  or  more :  a 
cube  of  copper,  on  the  contrary,  was  struck  almost 
instantly  motionless  when  the  circuit  was  established. 


XIV. 

PHYSICAL  BASIS   OF  SOLAR   CHEMISTBT? 

OMITTING-  all  preface,  attention  was  first  drawn  to 
an  experimental  arrangement  intended  to  prove 
that  gaseous  bodies  radiate  heat  in  different  degrees. 
Near  a  double  screen  of  polished  tin  was  placed  an 
ordinary  ring  gas-burner,  and  on  this  was  placed  a  hot 
copper  ball,  from  which  a  column  of  he.p.ted  air  ascended. 
Behind  the  screen,  but  so  situated  that  no  ray  from  the 
ball  could  reach  the  instrument,  was  an  excellent 
thermo-electric  pile,  connected  by  wires  with  a  very 
delicate  galvanometer.  The  pile  was  known  to  be  an 
instrument  whereby  heat  is  applied  to  the  generation 
of  electric  currents ;  the  strength  of  the  current  being 
an  accurate  measure  of  the  quantity  of  the  heat.  As 
long  as  both  faces  of  the  pile  are  at  the  same  tempera- 
ture, no  current  is  produced ;  but  the  slightest  difference 
in  the  temperature  of  the  two  faces  at  once  declares 
itself  by  the  production  of  a  current,  which,  when 
carried  through  the  galvanometer,  indicates  by  the 
deflection  of  the  needle  both  its  strength  and  its 
direction. 

The  two  faces  of  the  pile  were  in  the  first  instance 
brought  to  the  same  temperature ;  the  equilibrium 
being  shown  by  the  needle  of  the  galvanometer  standing 

*  Fiom  a  disconrse  delivered  at  the  Boyal  Institation  of  Oreat 
Britain,  June  7, 1861. 


330  FEAGMENTS  OF  SCIENCE. 

at  zero.  The  rays  emitted  by  the.  current  of  hot  air 
already  referred  to  were  permitted  to  fall  upon  one  of 
the  faces  of  the  pile ;  and  an  extremely  slight  move- 
ment of  the  needle  showed  that  the  radiation  from  the 
hot  air,  though  sensible,  was  extremely  feeble.  Con- 
nected with  the  ring-burner  was  a  holder  containing 
oxygen  gas ;  and  by  turning  a  cock,  a  stream  of  this 
gas  was  permitted  to  issue  from  the  burner,  strike  the 
copper  ball,  and  ascend  in  a  heated  column  in  front  of 
the  pile.  The  result  was,  that  oxygen  showed  itself,  as  a 
radiator  of  heat,  to  be  quite  as  feeble  as  atmospheric  air. 

A  second  holder  containing  defiant  gas  was  then 
connected  with  the  ring-burner.  Oxygen  and  air  had 
already  flowed  over  the  ball  and  cooled  it  in  some  degree. 
Hence  the  olefiant  gas  laboured  under  a  disadvantage. 
But  on  permitting  the  gas  to  rise  from  the  ball,  it  casts 
an  amount  of  heat  against  the  adjacent  face  of  the  pile 
sufficient  to  impel  the  needle  of  the  galvanometer  almost 
to  90°.  This  experiment  proved  the  vast  difference 
between  two  equally  invisible  gases  with  regard  to  their 
power  of  emitting  radiant  heat. 

The  converse  experiment  was  now  performed.  The 
thermo-electric  pile  was  removed  and  placed  between 
two  cubes  filled  with  water  kept  in  a  state  of  constant 
ebullition  ;  and  it  was  so  arranged  that  the  quantities 
of  heat  falling  from  the  cubes  on  the  opposite  faces  of 
the  pile  were  exactly  equal,  thus  neutralising  each 
other.  The  needle  of  the  galvanometer  being  at  zero, 
a  sheet  of  oxygen  gas  was  caused  to  issue  from  a  slit 
between  one  of  the  cubes  and  the  adjacent  face  of  the 
pile.  If  this  sheet  of  gas  possessed  any  sensible  power 
of  intercepting  the  thermal  rays  from  the  cube,  one 
face  of  the  pile  being  deprived  of  the  heat  thus  inter- 
cepted, a  difference  of  temperature  between  its  two 
fiaces  would  instantly  set  in,  and  the  result  would  be 


PHYSICAL  BASIS  OF  SOLAE  CHEMISTRY.         331 

declared  by  the  galvanometer.  The  quantity  absorbed 
by  the  oxygen  under  those  circumstances  was  too  feeble 
to  affect  the  galvanometer;  the  gas,  in  fact,  proved 
perfectly  transparent  to  the  rays  of  heat.  It  had  but 
a  feeble  power  of  radiation :  it  had  an  equally  feeble 
power  of  absorption. 

The  pile  remaining  in  its  position,  a  sheet  of  olefiant 
gas  was  caused  to  issue  from  the  same  slit  as  that  through 
which  the  oxygen  had  passed.  No  one  present  could 
see  the  gas ;  it  was  quite  invisible,  the  light  went 
through  it  as  freely  as  through  oxygen  or  air  ;  but  its 
effect  upon  the  thermal  rays  emanating  from  the  cube 
was  what  might  be  expected  from  a  sheet  of  metal.  A 
quantity  so  large  was  cut  off,  that  the  needle  of  the 
galvanometer,  promptly  quitting  the  zero  line,  moved 
with  energy  to  its  stops.  Thus  the  olefiant  gas,  so  light 
and  clear  and  pervious  to  luminous  rays,  was  proved  to 
be  a  most  potent  destroyer  of  the  rays  emanating  from 
an  obscure  source.  The  reciprocity  of  action  established 
in  the  case  of  oxygen  comes  out  here ;  the  good  radiator 
is  found  by  this  experiment  to  be  the  good  absorber. 

This  result,  now  exhibited  before  a  public  audience 
for  the  first  time,  was  typical  of  what  had  been  obtained 
with  gases  generally.  Groing  through  the  entire  list  of 
gases  and  vapours  in  this  way,  we  find  radiation  and 
absorption  to  be  as  rigidly  associated  as  positive  and 
negative  in  electricity,  or  as  north  and  south  polarity 
in  magnetism.  So  that  if  we  make  the  number  which 
expresses  the  absorptive  power  the  numerator  of  a 
fraction,  and  that  which  expresses  its  radiative  power 
the  denominator,  the  result  would  be,  that  on  account 
of  the  numerator  and  denominator  varying  in  the  same 
proportion,  the  value  of  that  fraction  would  always 
remain  the  same,  whatever  might  be  the  gas  or  vapour 
experimented  with. 


332  FEAGMENTS  OF  SCIENCE. 

But  why  should  this  reciprocity  exist?  What  is 
the  meaning  of  absorption  ?  what  is  the  meaning  of 
radiation?  When  you  cast  a  stone  into  still  water, 
rings  of  waves  surround  the  place  where  it  falls;  motion 
is  radiated  on  all  sides  from  the  centre  of  disturbance. 
When  a  hammer  strikes  a  bell,  the  latter  vibrates ;  and 
sound,  which  is  nothing  more  than  an  undulatory  motion 
of  the  air,  is  radiated  in  all  directions.  Modern  philo- 
sophy reduces  light  and  heat  to  the  same  mechanical 
category.  A  luminous  body  is  one  with  its  atoms  in 
a  state  of  vibration ;  a  hot  body  is  one  with  its  atoms 
also  vibrating,  but  at  a  rate  which  is  incompetent 
to  excite  the  sense  of  vision  ;  and,  as  a  sounding  body 
has  the  air  around  it,  through  which  it  propagates  its 
vibrations,  so  also  the  luminous  or  heated  body  has  a 
medium,  called  ether,  which  accepts  its  motions  and 
caiTies  them  forward  with  inconceivable  velocity. 
Kadiation,  then,  as  regards  both  light  and  heat,  is  the 
transference  of  motion  from  the  vibrating  body  to  the 
ether  in  which  it  swings :  and,  as  in  the  case  of  sound, 
the  motion  imparted  to  the  air  is  soon  transferred  to 
surrounding  objects,  against  which  the  aerial  undu- 
lations strike,  the  sound  being,  in  technical  language, 
abaorhed ;  so  also  with  regard  to  light  and  heat,  absorp- 
tion consists  in  the  transference  of  motion  from  the 
agitated  ether  to  the  molecules  of  the  absorbing  body. 

The  simple  atoms  are  found  to  be  bad  radiators ; 
the  compound  atoms  good  ones :  and  the  higher  the 
degree  of  complexity  in  the  atomic  grouping,  the  more 
potent,  as  a  general  rule,  is  the  radiation  and  absorption. 
Let  us  get  definite  ideas  here,  however  gross,  and  purify 
them  afterwards  by  the  process  of  abstraction.  Imagine 
our  simple  atoms  swinging  like  single  spheres  in  the 
ether ;  they  cannot  create  the  swell  which  a  group  of 
them  united  to  form  a  system  can  produce.     An  oar 


PHYSICAL  BASIS  OF  SOLAE  CHEMISTEY.         333 

runs  freely  edgeways  through  the  water,  and  imparts  far 
less  of  its  motion  to  the  water  than  when  its  broad  flat 
side  is  brought  to  bear  upon  it.  In  our  present  language 
the  oar,  broad  side  vertical,  is  a  good  radiator ;  broad 
Bide  horizontal,  it  is  a  bad  radiator.  Conversely  the 
waves  of  water,  impinging  upon  the  flat  face  of  the  oar- 
blade,  will  impart  a  greater  amount  of  motion  to  it  than 
when  impinging  upon  the  edge.  In  the  position  in 
which  the  oar  radiates  well,  it  also  absorbs  well.  Simple 
atoms  glide  through  the  ether  without  much  resistance; 
compound  ones  encounter  resistance,  and  hence  yield  up 
more  speedily  their  motion  to  the  ether.  Mix  oxygen 
and  nitrogen  mechanically,  they  absorb  and  radiate  a 
certain  amount  of  heat.  Cause  these  gases  to  combine 
chemically  and  form  nitrous  oxide,  both  the  absorption 
and  radiation  are  thereby  augmented  hundreds  of  times  I 
In  this  way  we  look  with  the  telescope  of  the  in- 
tellect into  atomic  systems,  and  obtain  a  conception  of 
processes  which  the  eye  of  sense  can  never  reach.  But 
gases  and  vapoiurs  possess  a  power  of  choice  as  to  the 
rays  which  they  absorb.  They  single  out  certain  groups 
of  rays  for  destruction,  and  allow  other  groups  to  pass 
unharmed.  This  is  best  illustrated  by  a  famous  experi- 
ment of  Sir  David  Brewster's,  modified  to  suit  present 
requirements.  Into  a  glass  cylinder,  with  its  ends 
stopped  by  discs  of  plate-glass,  a  small  quantity  of 
nitrous  acid  gas  is  introduced;  the  presence  of  the 
gas  being  indicated  by  its  rich  brown  colour.  The 
beam  from  an  electric  lamp  being  sent  through  two 
prisms  of  bisulphide  of  carbon,  a  spectrum  seven  feet 
long  and  eighteen  inches  wide  is  cast  upon  the  screen. 
Introducing  the  cylinder  containing  the  nitrous  acid 
into  the  path  of  the  beam  as  it  issues  from  the  lamp, 
the  splendid  and  continuous  spectrum  becomes  instantly 
furrowed  by  numerous  dark  bands,  the  rays  answering 


334  FRAGMENTS  OF  SCIENCE. 

to  which  are  intercepted  by  the  nitric  gas,  while  the 
light  which  falls  upon  the  intervening  spaces  is  per- 
mitted to  pass  with  comparative  impunity. 

Here  also  the  principle  of  reciprocity,  as  regards 
radiation  and  absorption,  holds  good  ;  and  could  we, 
without  otherwise  altering  its  physical  character,  render 
that  nitrous  gas  luminous,  we  should  find  that  the  very 
rays  which  it  absorbs  are  precisely  those  which  it  would 
emit.  When  atmospheric  air  and  other  gases  are  brought 
to  a  state  of  intense  incandescence  by  the  passage  of  an 
electric  spark,  the  spectra  which  we  obtain  from  them 
consist  of  a  series  of  bright  bands.  But  such  spectra 
are  produced  with  the  greatest  brilliancy  when,  instead 
of  ordinary  gases,  we  make  use  of  metals  heated  so 
highly  as  to  volatilise  them.  This  is  easily  done  by 
the  voltaic  current.  A  capsule  of  carbon  filled  with 
mercury,  which  formed  the  positive  electrode  of  the 
electric  lamp,  has  a  carbon  point  brought  down  upon 
it.  On  separating  the  one  from  the  other,  a  brilliant 
arc  containing  the  mercury  in  a  volatilised  condition 
passes  between  them.  The  spectrum  of  this  arc  is 
not  continuous  like  that  of  the  solid  carbon  points, 
but  consists  of  a  series  of  vivid  bands,  each  corre- 
sponding in  colour  to  that  particular  portion  of  the 
spectrum  to  which  its  rays  belong.  Copper  gives  its 
system  of  bands;  zinc  gives  its  system;  and  brass,  which 
is  an  alloy  of  copper  and  zinc,  gives  a  spectrum  made 
up  of  the  bands  belonging  to  both  metals. 

Not  only,  however,  when  metals  are  united  like  zinc 
and  copper  to  form  an  alloy,  is  it  possible  to  obtain 
the  bands  which  belong  to  them.  No  matter  how 
we  may  disguise  the  metal—  allowing  it  to  unite  with 
oxygen  to  form  an  oxide,  and  this  again  with  an  acid  to 
form  a  salt ;  if  the  heat  applied  be  sufficiently  intense, 
the  bands  belonging  to  the  metal  reveal  themselves  with 


PHYSICAL  BASIS  OF  SOLAE  CHEMISTEY.         336 

perfect  definition.  Into  holes  drilled  in  a  cylinder  of 
retort  carbon,  pure  culinary  salt  is  introduced.  When 
the  carbon  is  made  the  positive  electrode  of  the 
lamp,  the  resultant  spectrum  shows  the  brilliant  yellow 
lines  of  the  metal  sodium.  Similar  experiments  made 
with  the  chlorides  of  strontium,  calcium,  lithium,*  and 
other  metals,  give  the  bands  due  to  the  respective 
metals.  When  different  salts  are  mixed  together,  and 
rammed  into  holes  in  the  carbon ;  a  spectrum  is  ob- 
tained which  contains  the  bands  of  them  all. 

The  position  of  these  bright  bands  never  varies,  and 
each  metal  has  its  own  system.  Hence  the  competent 
observer  can  infer  from  the  bands  of  the  spectrum  the 
metals  which  produce  it.  It  is  a  language  addressed  to 
the  eye  instead  of  tlie  ear  ;  and  the  certainty  would  not 
be  augmented  if  each  metal  possessed  the  power  of 
audibly  calling  out, '  I  am  here  1 '  Nor  is  this  language 
affected  by  distance.  If  we  find  that  the  sun  or  the 
stars  give  us  the  bands  of  our  terrestrial  metals,  it  is  a 
declaration  on  the  part  of  these  orbs  that  such  metals 
enter  into  their  composition.  Does  the  sun  give  us  any 
such  intimation  ?  Does  the  solar  spectrum  exhibit  bright 
lines  which  we  might  compare  with  those  produced  by 
our  terrestrial  metals,  and  prove  either  their  identity 
or  difference  ?  No.  The  solar  spectrum,  when  closely 
examined,  gives  us  a  multitude  of  fine  dark  lines  instead 
of  bright  ones.  They  were  first  noticed  by  Dr.  WoUas- 
ton,  but  were  multiplied  and  investigated  with  profound 
skill  by  Fraunhofer,  and  named  after  him  Fraunhofer's 

*  The  vividness  of  the  colours  of  the  lithium  spectrum  is  extra- 
ordinary; the  spectrum,  moreover,  contained  a  blue  band  of  in- 
describable splendour.  It  was  thought  by  many,  during  the 
discourse,  that  I  had  mistaken  strontium  for  lithium,  as  this  blue 
band  had  never  before  been  seen.  I  have  obtained  it  many  ti  mea 
since  ;  and  my  friend  Dr.  Miller,  having  kindly  analysed  the  sub- 
slaxice  made  use  of,  pronounces  it  pure  chloride  of  lithium. — J.  T. 


336  FRAGMENTS  OF  SCIENCE. 

lines.  They  had  been  long  a  standing  puzzle  to  philo- 
sophers. The  bright  lines  yielded  by  metallic  vapours 
had  been  also  known  to  us  for  years  ;  but  the  connec- 
tion between  both  classes  of  phenomena  was  wholly 
unknown,  until  Kirchhofif,  with  admirable  acuteness, 
revealed  the  secret,  and  placed  it  at  the  same  time  in 
our  power  to  chemically  analyse  the  sun. 

We  have  now  some  difficult  work  before  us.  Hither- 
to we  have  been  delighted  by  objects  which  addressed 
themselves  as  much  to  our  aesthetic  taste  as  to  our  scien- 
tific faculty  ;  we  have  ridden  pleasantly  to  the  base  of 
the  final  cone  of  Etna,  and  must  now  dismoimt  and 
march  through  ashes  and  lava,  if  we  would  enjoy  the 
prospect  from  the  summit.  Our  problem  is  to  connect 
the  dark  lines  of  Fraunhofer  with  the  bright  ones  of  the 
metals.  The  white  beam  of  the  lamp  is  refracted  in 
passing  through  our  two  prisms,  but  its  different  com- 
ponents are  refracted  in  different  degrees,  and  thus  its 
colours  are  drawn  apart.  Now  the  colour  depends  solely 
upon  the  rate  of  oscillation  of  the  atoms  of  the  lumi- 
nous body  ;  red  light  being  produced  by  one  rate,  blue 
light  by  a  much  quicker  rate,  and  the  colours  between 
red  and  blue  by  th?  intermediate  rates.  The  solid  in- 
candescent coal-points  give  us  a  continuous  spectrum  ; 
or  in  other  words  they  emit  rays  of  all  possible  periods 
between  the  two  extremes  of  the  spectrum.  Colour,  as 
many  of  you  know,  is  to  light  what  'pitch  is  to  sound. 
When  a  violin -player  presses  his  finger  on  a  string  he 
makes  it  shorter  and  tighter,  and  thus,  causing  it  to 
vibrate  more  speedily,  heightens  the  pitch.  Imagine 
such  a  player  to  move  his  fingers  slowly  along  the  string, 
shortening  it  gradually  as  he  draws  his  bow,  the  note 
would  rise  in  pitch  by  a  regular  gradation  ;  there  would 
be  no  gap  intervening  between  note  and  note.  Here 
we  have  the  analogue  to  the  continuous  spectrum,  whose 
colours  insensibly  blend  together  without  gap  or  inter- 


PHYSICAL  BASIS  OF  SOLAE  CHEMISTRY.         337 

ruption,  from  the  red  of  the  lowest  pitch  to  the  violet  of 
the  highest.  But  suppose  the  player,  instead  of  gradu- 
ally shortening  his  string,  to  press  his  finger  on  a  cer- 
tain point,  and  to  sound  the  corresponding  note  ;  then 
to  pass  on  to  another  point  more  or  less  distant,  and 
sound  its  note ;  then  to  another,  and  so  on,  thus  sounding 
particular  notes  separated  from  each  other  by  gaps  which 
correspond  to  the  intervals  of  the  string  passed  over ;  we 
should  then  have  the  exact  analogue  of  a  spectrum  com- 
posed of  separate  bright  bands  with  intervals  of  darkness 
between  them.  But  this,  though  a  perfectly  true  and  in- 
telligible analogy,  is  not  sufficient  for  our  purpose ;  we 
must  look  with  the  mind's  eye  at  the  oscillating  atoms  of 
the  volatilised  metal.  Figure  these  atoms  as  connected 
together  by  springs  of  a  certain  tension,  which,  if  the 
atoms  are  squeezed  together,  push  them  again  asunder, 
and  if  the  atoms  are  drawn  apart,  pull  them  again 
together,  causing  them,  before  coming  to  rest,  to  quiver 
for  a  certain  time  at  a  certain  definite  rate  determined 
by  the  strength  of  the  spring.  Now  the  volatilised  metal 
which  gives  us  one  bright  band  is  to  be  figured  as  having 
its  atoms  united  by  springs  all  of  the  same  tension,  its 
vibrations  are  all  of  one  kind.  The  metal  which  gives 
us  two  bands  may  be  figured  as  having  some  of  its  atoms 
united  by  springs  of  one  tension,  and  others  by  springs 
of  a  diflferent  tension.  Its  vibrations  are  of  two  distinct 
kinds ;  so  also  when  we  have  three  or  more  bands  we  are 
to  figure  as  many  distinct  sets  of  springs,  each  capable 
of  vibrating  in  its  own  particular  time  and  at  a  different 
rate  from  the  others.  If  we  seize  this  idea  definitely,  we 
shall  have  no  difficulty  in  dropping  the  metaphor  of 
springs,  and  substituting  for  it  mentally  the  forces  by 
which  the  atoms  act  upon  each  other.  Having  thus  far 
cleared  our  way,  let  us  make  another  effort  to  advance. 
A  heavy  ivory  ball  is  here  suspended  from  a  string, 
I  blow  against  this  ball ;  a  single  puff  of  my  breath 


338  FRAGMENTS  OF  SCIENCE. 

moves  it  a  little  way  from  its  position  of  rest ;  it  swings 
back  towards  me,  and  when  it  reaches  the  limit  of  its 
swing  I  puff  again.  It  now  swings  further ;  and  thus  by 
timing  the  pufifs  I  can  so  accumulate  their  action  as  to 
produce  oscillations  of  large  amplitude.  The  ivory  ball 
here  has  absorbed  the  motion  which  my  breath  com- 
municated to  the  air.  I  now  bring  the  ball  to  rest. 
Suppose,  instead  of  the  breath,  a  wave  of  air  to  strike 
against  it,  and  that  this  wave  is  followed  by  a  series  of 
others  which  succeed  each  other  exactly  in  the  same 
intervals  as  my  puffs ;  it  is  obvious  that  these  waves 
would  communicate  their  motion  to  the  ball  and  cause 
it  to  swing  as  the  puffs  did.  And  it  is  equally  manifest 
that  this  would  not  be  the  case  if  the  impulses  of  the 
waves  were  not  properly  timed ;  for  then  the  motion 
imparted  to  the  pendulum  by  one  wave  would  be  neutra- 
lised by  another,  and  there  could  not  be  the  accumula- 
tion of  effect  obtained  when  the  periods  of  the  waves 
correspond  with  the  periods  of  the  pendulum.  So  much 
for  the  particular  impulses  absorbed  by  the  pendulum. 
But  if  such  a  pendulum  set  oscillating  in  air  could  pro-, 
duce  waves  in  the  air,  it  is  evident  that  the  waves  it 
would  produce  would  be  of  the  same  period  as  those 
whose  motions  it  would  take  up  or  absorb  most  com- 
pletely, if  they  struck  against  it. 

Perhaps  the  most  curious  effect  of  these  timed 
impulses  ever  described  was  that  observed  by  a  watch- 
maker, named  Ellicott,  in  the  year  1741.  JHe  left  two 
clocks  leaning  against  the  same  rail;  one  of  them, 
which  we  may  call  A,  was  set  going ;  the  other,  B,  not. 
Some  time  afterwards  he  found,  to  his  surprise,  that  B 
was  ticking  also.  The  pendulums  being  of  the  same 
length,  the  shocks  imparted  by  the  ticking  of  A  to  the 
rail  against  which  both  clocks  rested  were  propagated 
to  B,  and  were  so  timed  as  to  set  B  going.     Other 


PHYSICAL  BASIS  OF  SOLAB  CHEMISTRY.         339 

curious  effects  were  at  the  same  time  observed.  When 
the  pendulums  differed  from  each  other  a  certain  amount, 
A  set  B  going,  but  the  reaction  of  B  stopped  A.  Then 
B  set  A  going,  and  the  reaction  of  A  stopped  B.  When 
the  periods  of  oscillation  were  close  to  each  other,  but 
still  not  quite  alike,  the  clocks  mutually  controlled 
each  other,  and  by  a  kind  of  compromise  they  ticked 
in  perfect  unison. 

But  what  has  all  this  to  do  with  our  present  subject? 
The  varied  actions  of  the  universe  are  all  modes  of 
motion ;  and  the  vibration  of  a  ray  claims  strict  brother- 
hood with  the  vibrations  of  our  pendulum.  Suppose 
ethereal  waves  striking  upon  atoms  which  oscillate  in 
the  same  periods  as  the  waves,  the  motion  of  the  waves 
will  be  absorbed  by  the  atoms ;  suppose  we  send  our 
beam  of  white  light  through  a  sodium  flame,  the  atoms 
of  that  flame  will  be  chiefly  affected  by  those  undula- 
tions which  are  synchronous  with  their  own  periods  of 
vibration.  There  will  be  on  the  part  of  those  particular 
rays  a  transference  of  motion  from  the  agitated  ether 
to  the  atoms  of  the  volatilised  metal,  which,  as  already 
defined,  is  absorption. 

The  experiment  justifying  this  conclusion  is  now  for 
the  first  time  tobe  made  before  a  public  audience.  I  pass 
a  beam  through  our  two  prisms,  and  the  spectrum  spreads 
its  colours  upon  the  screen.  Between  the  lamp  and  the 
prism  I  interpose  a  snapdragon  light.  Alcohol  and 
water  are  here  mixed  with  common  salt,  and  the  metal 
dish  that  holds  them  is  heated  by  a  spirit-lamp.  The 
vapour  from  the  mixture  ignites  and  we  have  a  mono- 
chromatic flame.  Through  this  flame  the  beam  from  the 
lamp  is  now  passing ;  and  observe  the  result  upon  the 
spectrum.  You  see  a  shady  band  cut  out  of  the  yellow, 
— not  very  dark,  but  sufficiently  so  to  be  seen  by  every- 
body present. 


340  FRAGMENTS  OF  SCIENCE. 

But  let  me  exalt  this  effect.  Placing  in  front  of 
the  electric  lamp  the  intense  flame  of  a  large  Bunsen's 
burner,  a  platinum  capsule  containing  a  bit  of  sodium 
less  than  a  pea  in  magnitude  is  plunged  into  the  flame. 
The  sodium  soon  volatilises  and  burns  with  brilliant 
incandescence.  The  beam  crosses  the  flame,  and  at  the 
same  time  the  yellow  band  of  the  spectrum  is  clearly 
and  sharply  cut  out,  a  band  of  intense  darkness  occupy- 
ing its  place.  On  withdrawiug  the  sodium,  the  brilliant 
yellow  of  the  spectrum  takes  its  proper  place,  while  the 
reintroduction  of  the  flame  causes  the  band  to  reappear. 

Let  me  be  more  .precise  : — The  yellow  colour  of  the 
spectrum  extends  over  a  sensible  space,  blending  on  one 
side  with  the  orange  and  on  the  other  with  the  green. 
The  term  '  yellow  band  '  is  therefore  somewhat  indefi- 
nite. This  vagueness  may  be  entirely  removed.  By 
dipping  the  carbon-point  used  for  the  positive  electrode 
into  a  solution  of  common  salt,  and  replacing  it  in  the 
lamp,  the  bright  yellow  band  produced  by  the  sodium 
vapour  stands  out  from  the  spectrum.  When  tlie 
sodium  flame  is  caused  to  act  upon  the  beam  it  is  that 
particular  yellow  band  that  is  obliterated,  an  intensely 
black  streak  occupying  its  place. 

An  additional  step  of  reasoning  leads  to  the  con- 
clusion that  if,  instead  of  the  flame  of  sodium  alone,  we 
were  to  introduce  into  the  path  of  the  beam  a  flame  in 
which  lithium,  strontium,  magnesium,  calcium,  &c.,  are 
in  a  state  of  volatilisation,  each  metallic  vapour  would 
cut  out  a  system  of  bands,  corresponding  exactly  in 
position  with  the  bright  bands  of  the  same  metallic 
vapour.  The  light  of  our  electric  lamp  shining  through 
such  a  composite  flame  would  give  us  a  spectrum  cut 
up  by  dark  lines,  exactly  as  the  solar  spectrum  is  cut 
up  by  the  lines  of  Fraunhofer. 

Thus  by  the  combination  of  the  strictest  reasoning 


PHYSICAL  BASIS  OF  SOLAR  CHEMISTRY*         341 

with  the  most  conclusive  experiment,  we  reach  the 
solution  of  one  of  the  grandest  of  scientific  problems — 
the  constitution  of  the  sun.  The  sun  consists  of  a 
nucleus  surrounded  by  a  flaming  atmosphere.  The  light 
of  the  nucleus  would  give  us  a  continuous  spectrum, 
like  that  of  our  common  carbon-points  ;  but  having  to 
pass  through  the  photosphere,  as  our  beam  had  to  pass 
through  the  flame,  those  rays  of  the  nucleus  which  the 
photosphere  can  itself  emit  are  absorbed,  and  shaded 
spaces,  corresponding  to  the  particular  rays  absorbed, 
occur  in  the  spectrum.  Abolish  the  solar  nucleus,  and 
we  should  have  a  spectrum  showing  a  bright  line  in  the 
place  of  every  dark  line  of  Fraunhofer.  These  lines  are 
therefore  not  absolutely  dark,  but  dark  by  an  amount 
corresponding  to  the  difference  between  the  light  of 
the  nucleus  intercepted  by  the  photosphere,  and  the 
light  which  issues  from  the  latter. 

The  man  to  whom  we  owe  this  noble  generalisation 
is  Kirchhoflf,  Professor  of  Natural  Philosophy  in  the 
University  of  Heidelberg  ;  *  but,  like  every  other  great 
discovery,  it  is  compounded  of  various  elements.  Mr. 
Talbot  observed  the  bright  lines  in  the  spectra  of 
coloured  flames.  Sixteen  years  ago  Dr.  Miller  gave 
drawings  and  descriptions  of  the  spectra  of  various 
coloured  flames.  Wheatstone,  with  his  accustomed 
ingenuity,  analysed  the  li^ht  of  the  electric  spark,  and 
showed  that  the  metals  between  which  the  spark  passed 
determined  the  bright  bands  in  the  spectrum  of  the 
spark.  Masson  published  a  prize  essay  on  these  bands; 
Van  der  Willigen,  and  more  recently  Pliicker,  have 
given  us  beautiful  drawings  of  the  spectra,  obtained 
from  the  discharge  of  Kuhmkorfif's  coil.  But  none  of 
these  distinguished  men  betrayed  the  least  knowledge 
of  the  connection  between  the  bright  bands  of  the 
'  Now  Professor  in  the  University  of  Berlin. 


342  FRAGMENTS  OF  SCIENCE. 

metals  and  the  dark  lines  of  the  solar  spectrum.  The 
man  who  came  nearest  to  the  philosophy  of  the  subject 
was  Angstrom.  In  a  paper  translated  from  Poggen- 
dorfF's  *  Annalen '  by  myself,  and  published  in  the 
'Philosophical  Magazine'  for  1855,  he  indicates  that 
the  rays  which  a  body  absorbs  are  precisely  those  which 
it  can  emit  when  rendered  luminous.  In  another  place, 
he  speaks  of  one  of  his  spectra  giving  the  general  im- 
pression of  a  reversal  of  the  solar  spectrum.  Foucault, 
Stokes,  and  Thomson,  have  all  been  very  close  to  the 
discovery ;  and,  for  my  own  part,  the  examination  of 
the  radiation  and  absorption  of  heat  by  gases  and 
vapours,  some  of  the  results  of  which  I  placed  before 
you  at  the  commencement  of  this  discourse,  would  have 
led  me  in  1859  to  the  law  on  which  all  Kirchhoff's 
speculations  are  founded,  had  not  an  accident  withdrawn 
me  from  the  investigation.  But  Kirchhoflf's  claims  are 
unafifected  by  these  circumstances.  True,  much  that  I 
have  referred  to  formed  the  necessary  basis  of  his  dis- 
covery ;  so  did  the  laws  of  Kepler  furnish  to  Newton 
the  basis  of  the  theory  of  gravitation.  But  what 
Kirchhofif  has  done  carries  us  far  beyond  all  that  had 
before  been  accomplished.  He  has  introduced  the  order 
of  law  amid  a  vast  assemblage  of  empirical  observa- 
tions, and  has  ennobled  our  previous  knowledge  by 
showing  its  relationship  to  some  of  the  most  sublime  of 
natural  phenomena. 


XV. 

ELEMENTARY    MAGNETISM, 

A  LECTURE  TO   SCHOOLMASTERS. 

WE  have  no  reason  to  believe  that  the  sheep  or  the 
dog,  or  indeed  any  of  the  lower  animals,  feel  an 
interest  in  the  laws  by  which  natural  phenomena  are 
regulated.  A  herd  may  be  terrified  by  a  thunder- 
storm ;  birds  may  go  to  roost,  and  cattle  return  to 
their  stalls,  during  a  solar  eclipse  ;  but  neither  birds 
nor  cattle,  as  far  as  we  know,  ever  think  of  enquiring 
into  the  causes  of  these  things.  It  is  otherwise  with 
man.  The  presence  of  natural  objects,  the  occurrence 
of  natural  events,  the  varied  appearances  of  the  universe 
in  which  he  dwells  penetrate  beyond  his  organs  of  sense, 
and  appeal  to  an  inner  power  of  which  the  senses  are 
the  mere  instruments  and  excitants.  No  fact  is  to  him 
either  original  or  final.  He  cannot  limit  himself  to  the 
contemplation  of  it  alone,  but  endeavours  to  ascertain 
its  position  in  a  series  to  which  uniform  experience 
assures  him  it  must  belong.  He  regards  all  that  he 
witnesses  in  the  present  as  the  efflux  and  sequence  of 
something  that  has  gone  before,  and  as  the  source  of 
a  system  of  events  which  is  to  follow.  The  notion  of 
spontaneity,  by  which  in  his  ruder  state  he  accounted 
for  natural  events,  is  abandoned  ;  the  idea  that  nature 
is  an  aggregate  of  independent  parts  also  disappears,  as 
the  connection  and  mutual  dependence  of  physical 
23 


344  FKAGMENTS  OF  SCIENCE. 

powers  become  more  and  more  manifest:  until  he  is 
finally  led  to  regard  Nature  as  an  organic  whole — as  a 
body  each  of  whose  members  sympathises  with  the  rest, 
changing,  it  is  true,  from  age  to  age,  but  changing 
without  break  of  continuity  in  the  relation  of  cause 
and  effect. 

The  system  of  things  which  we  call  Nature  is,  how- 
ever, too  vast  and  various  to  be  studied  first-hand  by 
any  single  mind.  As  knowledge  extends  there  is  always 
a  tendency  to  subdivide  the  field  of  investigation.  Its 
various  parts  are  taken  up  by  different  minds,  and  thus 
receive  a  greater  amount  of  attention  than  could  pos- 
sibly be  bestowed  on  them  if  each  investigator  aimed 
at  the  mastery  of  the  whole.  The  centrifugal  form  in 
which  knowledge,  as  a  whole,  advances,  spreading  ever 
wider  on  all  sides,  is  due  in  reality  to  the  exertions  of 
individuals,  each  of  whom  directs  his  efforts,  more  or 
less,  along  a  single  line.  Accepting,  in  many  respects, 
his  culture  from  his  fellow-men — taking  it  from  spoken 
words  or  from  written  books — in  some  one  direction, 
the  student  of  Nature  ought  actually  to  touch  his  work. 
He  may  otherwise  be  a  distributor  of  knowledge,  but 
not  a  creator,  and  he  fails  to  attain  that  vitality  of 
thought,  and  correctness  of  judgment,  which  direct 
and  habitual  contact  with  natural  truth  can  alone  im- 
part. 

One  large  department  of  the  system  of  Nature 
which  forms  the  chief  subject  of  my  own  studies,  and 
to  which  it  is  my  duty  to  call  your  attention  this 
evening,  is  that  of  physics,  or  natural  philosophy.  This 
term  is  large  enough  to  cover  the  study  of  Nature  gene- 
rally, but  it  is  usually  restricted  to  a  department  which, 
perhaps,  lies  closer  to  our  perceptions  than  any  other. 
It  deals  with  the  phenomena  and  laws  of  light  and 
heat — with  the  phenomena  and  laws  of  magnetism  and 


ELEMENTAEY  MAGNETISM.  345 

electricity — with  those  of  sound — with  the  pressures  and 
motions  of  liquids  and  gases,  whether  at  rest  or  in  a  state 
of  translation  or  of  undulation.  The  science  of  mechanics 
is  a  portion  of  natural  philosophy,  though  at  present 
so  large  as  to  need  the  exclusive  attention  of  him  who 
would  cultivate  it  profoundly.  Astronomy  is  the  ap- 
plication of  physics  to  the  motions  of  the  heavenly 
bodies,  the  vastness  of  the  field  causing  it,  however,  to 
be  regarded  as  a  department  in  itself.  In  chemistry 
physical  agents  play  important  parts.  By  heat  and 
light  we  cause  atoms  and  molecules  to  unite  or  to  fall 
asunder.  Electricity  exerts  a  similar  power.  Through 
their  ability  to  separate  nutritive  compounds  into  their 
constituents,  the  solar  beams  build  up  the  whole  vege- 
table world,  and  by  it  the  animal  world.  The  touch  of 
the  self- same  beams  causes  hydrogen  and  chlorine  to 
imite  with  sudden  explosion,  and  to  form  by  their  com- 
bination a  powerful  acid.  Thus  physics  and  chemistry 
intermingle.  Physical  agents  are,  however,  employed 
by  the  chemist  as  a  means  to  an  end  ;  while  in  physics 
proper  the  laws  and  phenomena  of  the  agents  them- 
selves, both  qualitative  and  quantitative,  are  the  pri- 
mary objects  of  attention. 

My  duty  here  to-night  is  to  spend  an  hour  in  telling 
how  this  subject  is  to  be  studied,  and  how  a  knowledge 
of  it  is  to  be  imparted  to  others.  From  the  domain  of 
physics,  which  would  be  unmanageable  as  a  whole,  T 
select  as  a  sample  the  subject  of  magnetism.  I  might 
readily  entertain  you  on  the  present  occasion  with  an 
account  of  what  natural  philosophy  has  accomplished. 
I  might  point  to  those  applications  of  science  of  which 
we  hear  so  much  in  the  newspapers,  and  which  are 
so  often  mistaken  for  science  itself.  I  might,  of  course, 
ring  changes  on  the  steam-engine  and  the  telegraph, 
the  electrotype  and  the  photograph,  the  medical  appli- 


346  FEAGMENTS  OF  SCIENCE. 

cations  of  physics,  and  the  various  other  inlets  by  which 
scientific  thought  filters  into  practical  life.  That  would 
be  easy  compared  with  the  task  of  informing  you  how 
you  are  to  make  the  study  of  physics  the  instrument  of 
your  pupil's  culture ;  how  you  are  to  possess  its  facts 
and  make  them  living  seeds  which  shall  take  root  and 
grow  in  the  mind,  and  not  lie  like  dead  lumber  in  the 
storehouse  of  memory.  This  is  a  task  much  heavier 
than  the  mere  recounting  of  scientific  achievements  ; 
and  it  is  one  which,  feeling  my  own  want  of  time  to 
execute  it  aright,  I  might  well  hesitate  to  accept. 

But  let  me  sink  excuses,  and  attack  the  work  before 
me.  First  and  foremost,  then,  I  would  advise  you  to 
get  a  knowledge  of  facts  from  actual  observation. 
Facts  looked  at  directly  are  vital ;  when  they  pass  into 
words  half  the  sap  is  taken  out  of  them.  You  wish, 
for  example,  to  get  a  knowledge  of  magnetism ;  well, 
provide  yourself  with  a  good  book  on  the  subject,  if 
you  can,  but  do  not  be  content  with  what  the  book  tells 
you ;  do  not  be  satisfied  with  its  descriptive  woodcuts  ; 
see  the  operations  of  the  force  yourself.  Half  of  our 
book  writers  describe  experiments  which  they  never 
made,  and  their  descriptions  often  lack  both  force  and 
truth  ;  but,  no  matter  how  clever  or  conscientious  they 
may  be,  their  written  words  cannot  supply  the  place  of 
actual  observation.  Every  fact  has  numerous  radia- 
tions, which  are  shorn  off  by  the  man  who  describes  it. 
Gro,  then,  to  a  philosophical  instrument  maker,  and 
give  a  shilling  or  half  a  crown  for  a  straight  bar- 
magnet,  or,  if  you  can  afford  it,  purchase  a  pair  of 
them;  or  get  a  smith  to  cut  a  length  of  ten  inches 
from  a  bar  of  steel  an  inch  wide  and  half  an  inch 
thick ;  file  its  ends  smoothly,  harden  it,  and  get  some- 
body like  myself  to  magnetise  it.  Procure  some  darn- 
ing needles,  and  also  a  little  unspun  silk,  which  will 


ELEMENTARY  MAGNETISM.  347 

give  you  a  suspending  fibre  void  of  torsion.  Make  a 
little  loop  of  paper,  or  of  wire,  and  attach  your  fibre 
to  it.  Do  it  neatly.  In  the  loop  place  a  darning- 
needle,  and  bring  the  two  ends  or  poles,  as  they  are 
called,  of  your  bar-magnet  successively  up  to  the  ends 
of  the  needle.  Both  the  poles,  you  find,  attract  both 
ends  of  the  needle.  Eeplace  the  needle  by  a  bit  of 
annealed  iron  wire ;  the  same  effects  ensue.  Suspend 
successively  little  rods  of  lead,  copper,  silver,  brass, 
wood,  glass,  ivory,  or  whalebone ;  the  magnet  produces 
no  sensible  effect  upon  any  of  the  substances.  You 
thence  infer  a  special  property  in  the  case  of  steel  and 
iron.  Multiply  your  experiments,  however,  and  you 
will  find  that  some  other  substances,  besides  iron  and 
steel,  are  acted  upon  by  your  magnet.  A  rod  of  the 
metal  nickel,  or  of  the  metal  cobalt,  from  which  the 
blue  colour  used  by  painters  is  derived,  exhibits 
powers  similar  to  those  observed  with  the  iron  and  steel. 
In  studying  the  character  of  the  force  you  may, 
however,  confine  yourself  to  iron  and  steel,  which  are 
always  at  hand.  Make  your  experiments  with  the 
darning-needle  over  and  over  again ;  operate  on  both 
ends  of  the  needle  ;  try  both  ends  of  the  magnet.  Do 
not  think  the  work  dull ;  you  are  conversing  with 
Nature,  and  must  acquire  over  her  language  a  certain 
grace  and  mastery,  which  practice  can  alone  impart. 
Let  every  movement  be  made  with  care,  and  avoid 
slovenliness  from  the  outset.  Experiment,  as  I  have 
said,  is  the  language  by  which  we  address  Nature,  and 
through  which  she  sends  her  replies  ;  in  the  use  of  this 
language  a  lack  of  straightforwardness  is  as  possible, 
and  as  prejudicial,  as  in  the  spoken  language  of  the 
tongue.  If,  therefore,  you  wish  to  become  acquainted 
with  the  truth  of  Nature,  you  must  from  the  first  re- 
solve to  deal  with  her  sincerely. 


348  FRAGMENTS   OF  SCIENCE. 

Now  remove  your  needle  from  its  loop,  and  draw  it 
from  eye  to  point  along  one  of  the  ends  of  the  magnet ; 
resuspend  it,  and  repeat  your  former  experiment.  You 
now  find  that  each  extremity  of  the  magnet  attracts  one 
end  of  the  needle,  and  repels  the  other.  The  simple 
attraction  observed  in  the  first  instance,  is  now  replaced 
by  a  dual  force.  Eepeat  the  experiment  till  you  have 
thoroughly  observed  the  ends  which  attract  and  those 
which  repel  each  other. 

Withdraw  the  magnet  entirely  from  the  vicinity  of 
your  needle,  and  leave  the  latter  freely  suspended  by 
its  fibre.  Shelter  it  as  well  as  you  can  from  currents 
of  air,  and  if  you  have  iron  buttons  on  your  coat,  or  a 
steel  penknife  in  your  pocket,  beware  of  their  action. 
If  you  work  at  night,  beware  of  iron  candlesticks,  or 
of  brass  ones  with  iron  rods  inside.  Freed  from  such 
distiu-bances,  the  needle  takes  up  a  certain  determinate 
position.  It  sets  its  length  nearly  north  and  south. 
Draw  it  aside  and  let  it  go.  After  several  oscillations  it 
will  again  come  to  the  same  position.  If  you  have 
obtained  your  magnet  from  a  philosophical  instrument 
maker,  you  will  see  a  mark  on  one  of  its  ends.  Suppos- 
ing, then,  that  you  drew  your  needle  along  the  end  thus 
marked,  and  that  the  point  of  your  needle  was  the  last 
to  quit  the  magnet,  you  will  find  that  the  point  turns  to 
the  south,  the  eye  of  the  needle  turning  towards  the 
north.  Make  sure  of  this,  and  do  not  take  the  state- 
ment on  my  authority. 

Now  take  a  second  darning-needle  like  the  first, 
and  magnetise  it  in  precisely  the  same  manner :  freely 
suspended  it  also  will  turn  its  eye  to  the  north  and 
its  point  to  the  south.  Your  next  step  is  to  examine 
the  action  of  the  two  needles  which  you  have  thus  mag- 
netised upon  each  other. 

Take  one  of  them  in  your  hand,  and  leave  the  other 


ELEMENTAEY  MAGNETISM.  349 

Buspended ;  bring  the  eye-end  of  the  former  near  the 
eye-end  of  the  latter ;  the  "suspended  needle  retreats : 
it  is  repelled.  Make  the  same  experiment  with  the 
two  points ;  you  obtain  the  same  result,  the  suspended 
needle  is  repelled.  Now  cause  the  dissimilar  ends  to 
act  on  each  other — you  have  attraction — ^point  attracts 
eye,  and  eye  attracts  point.  Prove  the  reciprocity  of 
this  action  by  removing  the  suspended  needle,  and 
putting  the  other  in  its  place.  You  obtain  the  same 
result.  The  attraction,  then,  is  mutual,  and  the  repulsion 
is  mutual.  You  have  thus  demonstrated  in  the  clearest 
manner  the  fundamental  law  of  magnetism,  that  like 
poles  repel,  and  that  unlike  poles  attract,  each  other. 
You  may  say  that  this  is  all  easily  understood  without 
doing ;  but  do  it,  and  your  knowledge  will  not  be  con- 
fined to  what  I  have  uttered  here. 

I  have  said  that  one  end  of  your  bar  magnet  has  a 
mark  upon  it ;  lay  several  silk  fibres  together,  so  as  to 
get  sufficient  strength,  or  employ  a  thin  silk  ribbon, 
and  form  a  loop  large  enough  to  hold  your  magnet. 
Suspend  it ;  it  turns  its  marked  end  towards  the  north. 
This  marked  end  is  that  which  in  England  is  called  the 
north  pole.  If  a  common  smith  has  made  your  magnet, 
it  will  be  convenient  to  determine  its  north  pole  yourself, 
and  to  mark  it  with  a  file.  Vary  your  experiments  by 
causing  your  magnetised  darning-needle  to  attract  and 
repel  your  large  magnet ;  it  is  quite  competent  to  do 
so.  In  magnetising  the  needle,  I  have  supposed  the 
point  to  be  the  last  to  quit  the  marked  end  of  the 
magnet ;  the  point  of  the  needle  is  a  south  pole.  The 
end  which  last  quits  the  magnet  is  always  opposed  in 
polarity  to  the  end  of  the  magnet  with  which  it  has  been 
last  in  contact. 

You  may  perhaps  learn  all  this  in  a  single  hour :  but 
spend  several  at  it,  if  necessary ;  and  remember,  under* 


350  rRAGMENTS  OF  SCIENCE. 

standing  it  is  not  sufficient :  you  must  obtain  a  manual 
aptitude  in  addressing  Nature.  If  you  speak  to  your 
fellow-man  you  are  not  entitled  to  use  jargon.  Bad 
experiments  are  jargon  addressed  to  Nature,  and  just 
as  much  to  be  deprecated.  Manual  dexterity  in  illus- 
trating the  interaction  of  magnetic  poles  is  of  the 
utmost  importance  at  this  stage  of  your  progress ;  and 
you  must  not  neglect  attaining  this  power  over  your 
implements.  As  you  proceed,  moreover,  you  will  be 
tempted  to  do  more  than  I  can  possibly  suggest. 
Thoughts  will  occur  to  you  which  you  will  endeavour 
to  follow  out :  questions  will  arise  which  you  will  try 
to  answer.  The  same  experiment  may  be  twenty 
dififerent  things  to  twenty  people.  Having  witnessed 
the  action  of  pole  on  pole,  through  the  air,  you  will 
perhaps  try  whether  the  magnetic  power  is  not  to  be 
screened  off.  You  use  plates  of  glass,  wood,  slate, 
pasteboard,  or  gutta-percha,  but  find  them  all  pervious 
to  this  wondrous  force.  One  magnetic  pole  acts  upon 
another  through  these  bodies  as  if  they  were  not  present. 
Should  you  ever  become  a  patentee  for  the  regulation 
of  ships'  compasses,  you  will  not  fall,  as  some  projectors 
have  done,  into  the  error  of  screening  ofif  the  magnetism 
of  the  ship  by  the  interposition  of  such  substances. 

If  you  wish  to  teach  a  class  you  must  contrive  that 
the  effects  which  you  have  thus  far  witnessed  for  your- 
self shall  be  witnessed  by  twenty  or  thirty  pupils.  And 
here  your  private  ingenuity  must  come  into  play.  You 
will  attach  bits  of  paper  to  your  needles,  so  as  to  render 
their  movements  visible  at  a  distance,  denoting  the 
north  and  south  poles  by  different  colours,  say  green  and 
red.  You  may  also  improve  upon  your  darning-needle. 
Take  a  strip  of  sheet  steel,  heat  it  to  vivid  redness  and 
plunge  it  into  cold  water.  It  is  thereby  hardened ; 
rendered,  in  fact,  almost  as  brittle  as  glass.  Six  inches 
of  this,   magnetised  in  the  manner  of  the   darning- 


ELEMENTARY  MAGNETISM.  351 

needle,  will  be  better  able  to  carry  your  paper  indexes. 
Having^  secured  such  a  strip,  you  proceed  thus : — 

Magnetise  a  small  sewing-needle  and  determine  its 
poles ;  or,  break  half  an  inch,  or  an  inch,  off  your  magnet- 
ised darning-needle  and  suspend  it  by  a  fine  silk  fibre. 
The  sewing-needle,  or  the  fragment  of  the  darning 
needle,  is  now  to  be  used  as  a  test-needle,  to  examine 
the  distribution  of  the  magnetism  in  your  strip  of  steel. 
Hold  the  strip  upright  in  your  left  hand,  and  cause  the 
test-needle  to  approach  the  lower  end  of  your  strip ; 
one  end  of  the  test-needle  is  attracted,  the  other  is 
repelled.  Eaise  your  needle  along  the  strip  ;  its  oscil- 
lations, which  at  first  were  quick,  become  slower; 
opposite  the  middle  of  the  strip  they  cease  entirely ; 
neither  end  of  the  needle  is  attracted ;  above  the 
middle  the  test-needle  turns  suddenly  round,  its  other 
end  being  now  attracted.  Go  through  the  experi- 
ment thoroughly :  you  thus  learn  that  the  entire  lower 
half  of  the  strip  attracts  one  end  of  the  needle,  while 
the  entire  upper  half  attracts  the  opposite  end.  Sup- 
posing the  north  end  of  your  little  needle  to  be  that 
attracted  below,  you  infer  that  the  entire  lower  half  of 
your  magnetised  strip  exhibits  south  magnetism,  while 
the  entire  upper  half  exhibits  north  magnetism.  So 
far,  then,  you  have  determined  the  distribution  of 
magnetism  in  your  strip  of  steel. 

You  look  at  this  fact,  you  think  of  it ;  in  its  sug- 
gestiveness  the  value  of  an  experiment  chiefly  consists. 
The  thought  naturally  arises :  '  What  will  occur  if  I 
break  my  strip  of  steel  across  in  the  middle  ?  Shall  I 
obtain  two  magnets  each  possessing  a  single  pole?' 
Try  the  experiment ;  break  your  strip  of  steel,  and  test 
each  half  as  you  tested  the  whole.  The  mere  presenta- 
tion of  its  two  ends  in  succession  to  your  test-needle, 
suffices  to  show  that  you  have  not  a  magnet  with  a 


352  FRAGMENTS  OF  SCIENCE. 

single  pole — that  each  half  possesses  two  poles  with  a 
neutral  point  between  them.  And  if  you  again  break 
the  half  into  two  other  halves,  you  will  find  that  each 
quarter  of  the  original  strip  exhibits  precisely  the  same 
magnetic  distribution  as  the  whole  strip.  You  may 
continue  the  breaking  process :  no  matter  how  small 
your  fragment  may  be,  it  still  possesses  two  opposite 
poles  and  a  neutral  point  between  them.  Well,  your 
hand  ceases  to  break  where  breaking  becomes  a  mecha- 
nical impossibility ;  but  does  the  mind  stop  there  ? 
No :  you  follow  the  breaking  process  in  idea  when  you 
can  no  longer  realise  it  in  fact ;  your  thoughts  wander 
amid  the  very  atoms  of  your  steel,  and  you  conclude 
that  each  atom  is  a  magnet,  and  that  the  force  exerted 
by  the  strip  of  steel  is  the  mere  summation,  or  resultant, 
of  the  forces  of  its  ultimate  particles. 

Here,  then,  is  an  exhibition  of  power  which  we  can 
call  forth  at  pleasure  or  cause  to  disappear.  We  mag- 
netise our  strip  of  steel  by  drawing  it  along  the  pole  of 
a  magnet ;  we  can  demagnetise  it,  or  reverse  its  mag- 
netism, by  properly  drawing  it  along  the  same  pole  in 
the  opposite  direction.  What,  then,  is  the  real  nature 
of  this  wondrous  change  ?  What  is  it  that  takes  place 
among  the  atoms  of  the  steel  when  the  substance  ie 
magnetised  ?  The  question  leads  us  beyond  the  region 
of  sense,  and  into  that  of  imagination.  This  faculty, 
indeed,  is  the  di\'ining-rod  of  the  man  of  science. 
Not,  however,  an  imagination  which  catches  its  crea- 
tions from  the  air,  but  one  informed  and  inspired  by 
facts ;  capable  of  seizing  firmly  on  a  physical  image 
as  a  principle,  of  discerning  its  consequences,  and  of 
devising  rneans  whereby  these  forecasts  of  thought 
may  be  brought  to  an  experimental  test.  If  such  a 
principle  be  adequate  to  account  for  all  the  phenomena 
' — ^if  from  an  assumed  cause  the  observed  acts  neces- 


ELEMENTAEY  MAGNETISM.  353 

Hfirily  follow,  we  call  the  assumption  a  theory,  and, 
once  possessing  it,  we  can  not  only  revive  at  pleasure 
facts  already  known,  but  we  can  predict  others  which 
we  have  never  seen.  Thus,  then,  in  the  prosecution 
of  physical  science,  our  powers  of  observation,  memory, 
imagination,  and  inference,  are  all  drawn  upon.  We 
observe  facts  and  store  them  up  ;  the  constructive  imagi- 
nation broods  upon  these  memories,  tries  to  discern  their 
interdependence  and  weave  them  to  an  organic  whole. 
The  theoretic  principle  flashes  or  slowly  dawns  upon 
the  mind ;  and  then  the  deductive  faculty  interposes 
to  carry  out  the  principle  to  its  logical  consequences. 
A  perfect  theory  gives  dominion  over  natural  facts; 
and  even  an  assumption  which  can  only  partially  stand 
the  test  of  a  comparison  with  facts,  may  be  of  eminent 
use  in  enabling  us  to  connect  and  classify  groups  of 
phenomena.  The  theory  of  magnetic  fluids  is  of  this 
latter  character,  and  with  it  we  must  now  make  our- 
selves familiar. 

With  the  view  of  stamping  the  thing  more  firmly 
on  your  minds,  I  will  make  use  of  a  strong  and  vivid 
image.  In  optics,  red  and  green  are  called  comple- 
mentary colours ;  their  mixture  produces  white.  Now 
I  ask  you  to  imagine  each  of  these  colours  to  possess 
a  self-repulsive  power  ;  that  red  repels  red,  that  green 
repels  green ;  but  that  red  attracts  green  and  green 
attracts  red,  the  attraction  of  the  dissimilar  colours 
being  equal  to  the  repulsion  of  the  similar  ones. 
Imagine  the  two  colours  mixed  so  as  to  produce  white, 
and  suppose  two  strips  of  wood  painted  with  this  white ; 
what  will  be  their  action  upon  each  other  ?  Suspend 
one  of  them  freely  as  we  suspended  our  darning  needle, 
and  bring  the  other  near  it ;  what  will  occur  ?  The 
red  component  of  the  strip  you  hold  in  your  hand  will 
repel  the  red  component  of  your  suspended  strip  ;  but 


354  FKAGMENTS  OF  SCIENCE. 

then  it  will  attract  the  green,  and,  the  forces  being 
equal,  they  neutralise  each  other.  In  fact,  the  least 
reflection  shows  you  that  the  strips  will  be  as  indifferent 
to  each  other  as  two  unmagnetised  darning-needles 
would  be  under  the  same  circumstances. 

But  suppose,  instead  of  mixing  the  colours,  we 
painted  one  half  of  each  strip  from  centre  to  end  red,  and 
the  other  half  green,  it  is  perfectly  manifest  that  the 
two  strips  would  now  behave  towards  each  other  exactly 
as  our  two  magnetised  darning-needles — the  red  end 
wouldrepel  the  red  and  attract  the  green,the  green  would 
repel  the  green  and  attract  the  red  ;  so  that,  assuming 
two  colours  thus  related  to  each  other,  we  could  by 
their  mixture  produce  the  neutrality  of  an  immagnetised 
body,  while  by  their  separation  we  could  produce  the 
duality  of  action  of  magnetised  bodies. 

But  you  have  already  anticipated  a  defect  in  my 
conception ;  for  if  we  break  one  of  our  strips  of  wood 
in  the  middle  we  have  one  half  entirely  red,  and  the 
other  entirely  green,  and  with  these  it  would  be  im- 
possible to  imitate  the  action  of  our  broken  magnet. 
How,  then,  must  we  modify  our  conception?  We 
must  evidently  suppose  each  molecule  of  the  wood 
painted  green  on  one  face  and  red  on  the  opposite  one. 
The  resultant  action  of  all  the  atoms  would  then  exactly 
resemble  the  action  of  a  magnet.  Here  also,  if  the  two 
opposite  colours  of  each  atom  could  be  caused  to  mix 
so  as  to  produce  white,  we  should  have,  as  before,  per- 
fect neutrality. 

For  these  two  self-repellent  and  mutually  attractive 
colours,  substitute  in  your  minds  two  invisible  self- 
repellent  and  mutually  attractive  fluids,  which  in  ordi- 
nary steel  are  mixed  to  form  a  neutral  compound,  but 
which  the  act  of  magnetisation  separates  from  each 
other,  placing  the  opposite  fluids  on  the  opposite  face 


ELEMENTARY  MAGNETISM.  355 

of  eacli  molecule.  You  have  then  a  perfectly  distinct 
conception  of  the  celebrated  theory  of  magnetic  fluids. 
The  strength  of  the  magnetism  excited  is  supposed  to 
be  proportional  to  the  quantity  of  neutral  fluid  decom- 
posed. According  to  this  theory  nothing  is  actually 
transferred  from  the  exciting  magnet  to  the  excited 
steel.  The  act  of  magnetisation  consists  in  the  forcible 
separation  of  two  fluids  which  existed  in  the  steel  before 
it  was  magnetised,  but  which  then  neutralised  each 
other  by  their  coalescence.  And  if  you  test  your 
magnet,  after  it  has  excited  a  hundred  pieces  of  steel, 
you  will  find  that  it  has  lost  no  force — no  more,  in- 
deed, than  I  should  lose,  had  my  words  such  a  magnetic 
influence  on  your  minds  as  to  excite  in  them  a  strong 
resolve  to  study  natural  philosophy.  I  should  rather 
be  the  gainer  by  my  own  utterance,  and  by  the  reaction 
of  your  fervour.  The  magnet  also  is  the  gainer  by  the 
reaction  of  the  body  wliich  it  magnetises. 

Look  now  to  your  excited  piece  of  steel ;  figure 
each  molecule  with  its  opposed  fluids  spread  over  its 
opposite  faces.  How  can  this  state  of  things  be  per- 
manent ?  The  fluids,  by  hypothesis,  attract  each  other; 
what,  then,  keeps  them  apart?  Why  do  they  not 
instantly  rush  together  across  the  equator  of  the  atom,, 
and  thus  neutralise  each  other  ?  To  meet  this  question 
philosophers  have  been  obliged  to  infer  the  existence 
of  a  special  force,  which  holds  the  fluids  asunder.  They 
call  it  coercive  force  ;  and  it  is  found  that  those  kinds 
of  steel  which  ofier  most  resistance  to  beinof  magnetised 
— which  require  the  greatest  amount  of  '  coercion  '  to 
tear  their  fluids  asunder — are  the  very  ones  which  ofl*er 
the  greatest  resistance  to  the  reunion  of  the  fluids, 
after  they  have  been  once  separated.  Such  kinds  of 
steel  are  most  suited  to  the  formation  oi permanent 
magnets.     It  is   manifest,  indeed,  that  without  coer- 


356  FKAGMENTS  OF  SCIENCE. 

cive  force  a  permanent  magnet  would  not  be  at  all 
possible. 

Probably  long  before  this  you  will  have  dipped  the 
end  of  your  magnet  among  iron  filings,  and  observed 
how  they  cling  to  it;  or  into  a  nail-box,  and  found 
how  it  drags  the  nails  after  it.  I  know  very  well  that 
if  you  are  not  the  slaves  of  routine,  you  will  have  by 
this  time  done  many  things  that  I  have  not  told  you 
to  do,  and  thus  multiplied  your  experience  beyond  what 
I  have  indicated.  You  are  almost  sure  to  have  caused 
a  bit  of  iron  to  hang  from  the  end  of  your  magnet,  and 
you  have  probably  succeeded  in  causing  a  second  bit 
to  attach  itself  to  the  first,  a  third  to  the  second  ;  until 
finally  the  force  has  become  too  feeble  to  bear  the 
weight  of  more.  If  you  have  operated  with  nails,  you 
may  have  observed  that  the  points  and  edges  hold  to- 
gether with  the  greatest  tenacity ;  and  that  a  bit  of 
iron  clings  more  firmly  to  the  comer  of  your  magnet 
than  to  one  of  its  flat  surfaces.  In  short,  you  will  in 
all  likelihood  have  enriched  your  experience  in  many 
ways  without  any  special  direction  from  me. 

Well,  the  magnet  attracts  the  nail,  and  the  nail 
attracts  a  second  one.  This  proves  that  the  nail  in 
contact  with  the  magnet  has  had  the  magnetic  quality 
developed  in  it  by  that  contact.  If  it  be  withdrawn 
from  the  magnet  its  power  to  attract  its  fellow  nail 
ceases.  Contact,  however,  is  not  necessary.  A  sheet  of 
glass  or  paper,  or  a  space  of  air,  may  exist  between  the 
magnet  and  the  nail;  the  latter  is  still  magnetised, 
though  not  so  forcibly  as  when  in  actual  contact.  The 
nail  thus  presented  to  the  magnet  is  itself  a  temporary 
magnet.  That  end  which  is  turned  towards  the  mag- 
netic pole  has  the  opposite  magnetism  of  the  pole  which 
excites  it ;  the  end  most  remote  from  the  pole  has  the 
same  magnetism  as  the  pole  itself,  and  between  the 


ELEMENTAKY  MAGNETISM.  357 

two  poles  the  nail,  like  the  magnet,  possesses  a  mag- 
netic equator. 

Conversant  as  you  now  are  with  the  theory  of  mag- 
netic fluids,  you  have  already,  I  doubt  not,  anticipated 
me  in  imagining  the  exact  condition  of  an  iron  nail  under 
the  influence  of  the  magnet.  You  picture  the  iron  as 
possessing  the  neutral  fluid  in  abundance ;  you  picture 
the  magnetic  pole,  when  brought  near,  decomposing 
the  fluid  ;  repelling  the  fluid  of  a  like  kind  with  itself, 
and  attracting  the  unlike  fluid ;  thus  exciting  in  the 
parts  of  the  iron  nearest  to  itself  the  opposite  polarity. 
But  the  iron  is  incapable  of  becoming  a  permanent 
magnet.  It  only  shows  its  virtue  as  long  as  the  magnet 
acts  upon  it.  What,  then,  does  the  iron  lack  which  the 
steel  possesses  ?  It  lacks  coercive  force.  Its  fluids  are 
separated  with  ease  ;  but,  once  the  separating  cause  is 
removed,  they  flow  together  again,  and  neutrality  is 
restored.  Imagination  must  be  quite  nimble  in  pictur- 
ing these  changes — able  to  see  the  fluids  dividing  and 
reuniting,  according  as  the  magnet  is  brought  near  or 
withdrawn.  Fixing  a  definite  pole  in  your  mind,  you 
must  picture  the  precise  arrangement  of  the  two  fluids 
with  reference  to  this  pole,  and  be  able  to  arouse 
similar  pictures  in  the  minds  of  your  pupils.  You 
will  cause  them  to  place  magnets  and  iron  in  various 
positions,  and  describe  the  exact  magnetic  state  of  the 
iron  in  each  particular  case.  The  mere  facts  of  mag- 
netism will  have  their  interest  immensely  augmented 
by  an  acquaintance  with  the  principles  whereon  the  facts 
depend.  Still,  while  you  use  this  theory  of  magnetic  fluids 
to  track  out  the  phenomena  and  link  them  together, 
you  will  not  forget  to  tell  your  pupils  that  it  is  to  be  re- 
garded as  a  symbol  merely, — a  symbol,  moreover,  which 
is  incompetent  to  cover  all  the  facts,*  but  which  does  good 
*  This  theory  breaks  down  when  applied  to  diamagnetic  bodieg 


358  FEAGMENTS  OF  SCIENCE. 

practical  service  whilst  we  are  waiting  for  tlie  actual 
truth. 

The  state  of  excitement  into  which  iron  is  thrown 
by  the  influence  of  a  magnet,  is  sometimes  called 
'  magnetisation  by  influence.'  More  commonly,  how- 
ever, the  magnetism  is  said  to  be  '  induced '  in  the  iron, 
and  hence  this  mode  of  magnetising  is  called. 'mag 
netic  induction.'  Now,  there  is  nothing  theoretically 
perfect  in  Nature :  there  is  no  iron  so  soft  as  not  to 
possess  a  certain  amount  of  coercive  force,  and  no  steel 
so  hard  as  not  to  be  capable,  in  some  degree,  of  mag- 
netic induction.  Tlie  quality  of  steel  is  in  some 
measure  possessed  by  iron,  and  the  quality  of  iron  is 
shared  in  some  degree  by  steel.  It  is  in  virtue  of  this 
latter  fact  that  the  unmagnetised  darning-needle  was 
attracted  in  your  first  experiment ;  and  from  this  you 
may  at  once  deduce  the  consequence  that,  after  the 
steel  has  been  magnetised,  the  repulsive  action  of  a 
magnet  must  be  always  less  than  its  attractive  action. 
For  the  repulsion  is  opposed  by  the  inductive  action  of 
the  magnet  on  the  steel,  while  the  attraction  is  assisted 
by  the  same  inductive  action.  Make  this  clear  to  your 
minds,  and  verify  it  by  your  experiments,  "  In  some 
cases  you  can  actually  make  the  attraction  due  to  the 
temporary  magnetism  overbalance  the  repulsion  due  to 
the  permanent  magnetism,  and  thus  cause  two  poles  of 
the  same  kind  apparently  to  attract  each  other.  When, 
however,  good  hard  magnets  act  on  each  other  from  a 
sufficient  distance,  the  inductive  action  practically 
vanishes,  and  the  repulsion  of  like  poles  is  sensibly 
equal  to  the  attraction  of  unlike  ones. 

which  are  repelled  by  magnets.  Like  soft  iron,  such  bodies  are 
thrown  into  a  state  of  temporary  excitement,  in  virtue  of  which 
they  are  repelled ;  but  any  attempt  to  explain  such  a  repulsion  by 
the  decomposition  of  a  fluid  will  demonstrate  its  own  futility. 


ELEMENTARY  MAGNETISM.  359 

I  dwell  thus  long  on  elementary  principles,  because 
they  are  of  the  first  importance,  and  it  is  the  temptation 
of  this  age  of  unhealthy  cramming  to  neglect  them. 
Now  follow  me  a  little  farther.  In  examining  the 
distribution  of  magnetism  in  your  strip  of  steel  you 
raised  the  needle  slowly  from  bottom  to  top,  and  found 
what  we  called  a  neutral  point  at  the  centre.  Now 
does  the  magnet  really  exert  no  influence  on  the  pole 
presented  to  its  centre  ?     Let  us  see. 

Let  s  N,  fig.  11 ,  be  our  magnet,  and  let  n  represent 
a  particle  of  north  magnetism  placed  exactly  opposite 
the  middle  of  the  magnet.  Of  course  this  is  an  ima- 
ginary case,  as  you  can  never  in  reality  thus  detach 
your  north  magnetism  from  its  neighboui:.  But  sup- 
posing us  to  have  done  so,  what  would  be  the  action  of 
the  two  poles  of  the  magnet  on  n  ?  Your  reply  will  of 
course  be  that  the  pole  s  attracts  n  while  the  pole  n 
repels  it.  Let  the  magnitude  and  direction  of  the 
attraction  be  expressed  by  the  line  n  m,  and  the  mag- 


Fig.  11. 

nitude  and  direction  of  the  repulsion  by  the  line  n  o. 
Now,  the  particle  n  being  equally  distant  from  s  and  N, 
the  line  n  o,  expressing  the  repulsion,  will  be  equal  to 
m  91,  which  expresses  the  attraction.  Acted  upon  by 
two  such  forces,  the  particle  n  must  evidently  move  in 
the  direction  njp,  exactly  midway  between  mn  and  no. 
Hence  you  see  that,  although  there  is  no  tendency  of 
24 


360  FRAGMENTS  OF  SCIENCE. 

the  particle  n  to  move  towards  the  magnetic  equator, 
there  is  a  tendency  on  its  part  to  move  parallel  to  the 
magnet.  If,  instead  of  a  particle  of  north  magnetism, 
we  placed  a  particle  of  south  magnetism  opposite  to 
the  magnetic  equator,  it  would  evidently  be  urged 
along  the  line  nq\  and  if,  instead  of  two  separate 
particles  of  magnetism,  we  place  a  little  magnetic 
needle,  containing  both  north  and  south  magnetism, 
opposite  the  magnetic  equator,  its  south  pole  being 
urged  along  n  g,  and  its  porth  along  n  p,  the  little 
needle  will  be  compelled  to  set  itself  parallel  to  the 
magnet  s  N.  Make  the  experiment,  and  satisfy  your- 
selves that  this  is  a  true  deduction. 

Substitute  for  your  magnetic  needle  a  bit  of  iron 
wire,  devoid  of  permanent  magnetism,  and  it  will  set 
itself  exactly  as  the  needle  does.  Acted  upon  by  the 
magnet,  the  wire,  as  you  know,  becomes  a  magnet  and 
behaves  as  such ;  it  will  turn  its  north  pole  towards  p, 
and  south  pole  towards  q,  just  like  the  needle. 

But  supposing  you  shift  the  position  of  your  particle 
of  north  magnetism,  and  bring  it  nearer  to  one  end  of 
your  magnet  than  to  the  other ;  the  forces  acting  on 
the  particle  are  no  longer  equal ;  the  nearest  pole  of 


Fig.  12. 


the  magnet  will  act  more  powerfully  on  the  particle 
than  the  more  distant  one.  Let  s  N,  fig.  12,  be  the 
magnet,  and  n  the  particle   of  north  magnetism,  in  its 


ELEMENTAKY  MAGNETISM.  361 

new  position.  It  is  repelled  by  n,  and  attracted  by  s. 
Let  the  repulsion  be  represented  in  magnitude  and 
direction  by  the  line  n  o,  and  the  attraction  by  the 
shorter  line  n  m.  The  resultant  of  these  two  forces 
will  be  found  by  completing  the  parallelogram  Tnnop, 
and  drawing  its  diagonal  n  p.  Along  n  p,  then,  a 
particle  of  north  magnetism  would  be  urged  by  the 
simultaneous  action  of  s  and  n.  Substituting  a  particle 
of  south  magnetism  for  n,  the  same  reasoning  would 
lead  to  the  conclusion  that  the.  particle  would  be  urged 
along  n  q.  If  we  place  at  n  a  short  magnetic  needle, 
its  north  pole  will  be  urged  along  n  p, '  its  south  pole 
along  n  q^  the  only  position  possible  to  the  needle, 
thus  acted  on,  being  along  the  line  p  q,  which  is  no 
longer  parallel  to  the  magnet.  Verify  this  deduction 
by  actual  experiment. 

In  this  way  we  might  go  round  the  entire  magnet ; 
and,  considering  its  two  poles  as  two  centres  from 
which  the  force  emanates,  we  could,  in  accordance  with 
ordinary  mechanical  principles,  assign  a  definite  direc- 
tion to  the  magnetic  needle  at  every  particular  place. 
And  substituting,  as  before,  a  bit  of  iron  wire  for  the 
magnetic  needle,  the  positions  of  both  will  be  the 
same. 

Now,  I  think,  without  further  preface,  you  will  be 
able  to  comprehend  for  yourselves,  and  explain  to 
others,  one  of  the  most  interesting  effects  in  the  whole 
domain  of  magnetism.  Iron  filings  you  know  are 
particles  of  iron,  irregular  in  shape,  being  longer  in 
some  directions  than  in  otbers.  For  the  present  ex- 
periment, moreover,  instead  of  the  iron  filings,  very 
small  scraps  of  thin  iron  wire  might  be  employed.  I 
place  a  sheet  of  paper  over  the  magnet ;  it  is  all  the 
better  if  the  paper  be  stretched  on  a  wooden  frame,  as 
this  enables  us  to  keep  it  quite  level.     I  scatter  the 


Fig.  13. 


MAGNETIC  LINES    OF   FORCE. 
From  a  Photograph  by  Professor  Mateb. 


ELEJMENTAEY  MAGNETISM.  363 

filings,  or  the  scraps  of  wire,  from  a  sieve  upon  the 
paper,  and  tap  the  latter  gently,  so  as  to  liberate  the 
particles  for  a  moment  from  its  friction.  The  magnet 
acts  on  the  filings  through  the  paper,  and  see  how  it 
arranges  them  I  They  embrace  the  magnet  in  a  series 
of  beautiful  curves,  which  are  technically  called  *  mag- 
netic curves,*  or  *  lines  of  magnetic  force.'  Does  the 
meaning  of  these  lines  yet  flash  upon  you  ?  Set  your 
magnetic  needle,  or  your  suspended  bit  of  wire,  at  any 
point  of  one  of  the  curves,  and  you  will  find  the  direc- 
tion of  the  needle,  or  of  the  wire,  to  be  exactly  that  of 
the  particle  of  iron,  or  of  the  magnetic  curve,  at  that 
point.  Go  round  and  round  the  magnet ;  the  direction 
of  your  needle  always  coincides  with  the  direction  of 
the  curve  on  which  it  is  placed.  These,  then,  are  the 
lines  along  which  a  particle  of  south  magnetism,  if  you 
could  detach  it,  would  move  to  the  north  pole,  and  a 
bit  of  north  magnetism  to  the  south  pole.  They  are 
the  lines  along  which  the  decomposition  of  the  neutral 
fluid  takes  place.  In  the  case  of  the  magnetic  needle, 
one  of  its  poles  being  urged  in  one  direction,  and  the 
other  pole  in  the  opposite  direction,  the  needle  must 
necessarily  set  itself  as  a  tangent  to  the  curve.  I  will 
not  seek  to  simplify  this  subject  further.  If  there  be 
anything  obscure  or  confused  or  incomplete  in  my 
statement,  you  ought  now,  by  patient  thought,  to  be 
able  to  clear  away  the  obscurity,  to  reduce  the  con- 
fusion to  order,  and  to  supply  what  is  needed  to  render 
the  explanation  complete.  Do  not  quit  the  subject 
until  you  thoroughly  understand  it ;  and  if  you  are 
then  able  to  look  with  your  mind's  eye  at  the  play  of 
forces  around  a  magnet,  and  see  distinctly  the  operation 
of  those  forces  in  the  production  of  the  magnetic  curves, 
the  time  which  we  have  spent  together  will  not  have 
been  spent  in  vain. 


364  FRAaMENTS  OF  SCIENCE. 

In  this  thorough  manner  we  must  master  our  ma- 
terials, reason  upon  them,  and,  by  determined  study, 
attain  to  clearness  of  conception.  Facts  thus  dealt 
with  exercise  an  expansive  force  upon  the  intellect ; — 
they  widen  the  mind  to  generalisation.  We  soon 
recognise  a  brotherhood  between  the  larger  phenomena 
of  Nature  and  the  minute  efifects  which  we  have  ob- 
served in  our  private  chambers.  Why,  we  enquire, 
does  the  maofuetic  needle  set  north  and  south  ?  Evi- 
dently  it  is  compelled  to  do  so  by  the  earth  ;  the  great 
globe  which  we  inherit  is  itself  a  magnet.  Let  us 
learn  a  little  more  about  it.  By  means  of  a  bit  of 
wax,  or  otherwise,  attach  the  end  of  your  silk  fibre  to 
the  middle  point  of  your  magnetic  needle  ;  the  needle 
will  thus  be  uninterfered  with  by  the  paper  loop,  and 
will  enjoy  to  some  extent  a  power  of  ^dipping'  its 
point,  or  its  eye,  below  the  horizon.  Lay  your  bar 
magnet  on  a  table,  and  hold  the  needle  over  the  equator 
of  the  maofnet.  The  needle  sets  horizontal.  Move  it 
towards  the  north  end  of  the  magnet ;  the  south  end 
of  the  needle  dips,  the  dip  augmenting  as  you  approach 
the  north  pole,  over  which  the  needle,  if  free  to  move, 
will  set  itself  exactly  vertical.  Move  it  back  to  the 
centre,  it  resumes  its  horizontality ;  pass  it  on  towards 
the  south  pole,  its  north  end  now  dips,  and  directly 
over  the  south  pole  the  needle  becomes  vertical,  its 
north  end  being  now  turned  downwards.  Thus  we 
learn  that  on  the  one  side  of  the  magnetic  equator  the 
north  end  of  the  needle  dips;  on  the  other  side  the 
south  end  dips,  the  dip  varying  from  nothing  to  90°. 
If  we  go  to  the  equatorial  regions  of  the  earth  with  a 
suitably  suspended  needle  we  shall  find  there  the 
position  of  the  needle  horizontal.  If  we  sail  north  one 
end  of  the  needle  dips ;  if  we  sail  south  the  opposite 
end  dips ;    and  over   the   north    or   south   terrestrial 


ELEMENTAEY   MAGNETISM.  365 

magnetic  pole  the  needle  sets  vertical.  The  south 
magnetic  pole  has  not  yet  been  found,  but  Sir  James 
Koss  discovered  the  north  magnetic  pole  on  June  1, 
1831.  In  this  manner  we  establish  a  complete  paral- 
lelism between  the  action  of  the  earth  and  that  of  an 
ordinary  magnet. 

The  terrestrial  magnetic  poles  do  not  coincide  with 
the  geographical  ones ;  nor  does  the  earth's  magnetic 
equator  quite  coincide  with  the  geographical  equator. 
The  direction  of  the  magnetic  needle  in  London,  which 
is  called  the  magnetic  meridian,  encloses  an  angle  of  24** 
with  the  astronomical  meridian,  this  angle  being  called 
the  Declination  of  the  needle  for  London.  The  north 
pole  of  the  needle  now  lies  to  the  west  of  the  true  meri- 
dian ;  the  declination  is  westerly.  In  the  year  1660, 
however,  the  declination  was  nothing,  while  before  that 
time  it  was  easterly.  All  this  proves  that  the  earth's 
magnetic  constituents  are  gradually  changing  their  dis- 
tribution. This  change  is  very  slow  :  it  is  therefore 
called  the  secular  change,  and  the  observation  of  it  has 
not  yet  extended  over  a  sufficient  period  to  enable  us  to 
guess,  even  approximately,  at  its  laws. 

Having  thus  discovered,  to  some  extent,  the  secret  of 
the  earth's  magnetic  power,  we  can  turn  it  to  account.  In 
the  line  of  '  dip  '  I  hold  a  poker  formed  of  good  soft  iron. 
The  earth,  acting  as  a  magnet,  is  at  this  moment  con- 
straining the  two  fluids  of  the  poker  to  separate,  making 
the  lower  end  of  the  poker  a  north  pole,  and  the  upper  end 
a  south  pole.  Mark  the  experiment :  When  the  knob 
is  uppermost,  it  attracts  the  north  end  of  a  magnetic 
needle ;  when  undermost  it  attracts  the  south  end  of 
a  magnetic  needle.  With  such  a  poker  repeat  this  ex- 
periment and  satisfy  yourselves  that  the  fluids  shift  their 
position  according  to  the  manner  in  which  the  poker  is 
presented  to  the  earth.     It  has  already  been  stated  that 


366  FRAGMENTS  OF  SCIENCE. 

the  softest  iron  possesses  a  certain  amount  of  coercive 
force.  The  earth,  at  this  moment,  finds  in  this  force 
an  antagonist  which  opposes  the  decomposition  of  the 
neutral  fluid,  The  component  fluids  may  be  figured  as 
meeting  an  amount  of  friction,  or  possessing  an  amount 
of  adhesion,  which  prevents  them  from  gliding  over  the 
molecules  of  the  poker.  Can  we  assist  the  earth  in  this 
case  ?  If  we  wish  to  remove  the  residue  of  a  powder 
from  the  interior  surface  of  a  glass  to  which  the  powder 
clings,  we  invert  the  glass,  tap  it,  loosen  the  hold  of 
the  powder,  and  thus  enable  the  force  of  gravity  to  pull 
it  down.  So  also  by  tapping  the  end  of  the  poker  we 
loosen  the  adhesion  of  the  magnetic  fluids  to  the  mole- 
cules and  enable  the  earth  to  pull  them  apart.  But, 
what  is  the  consequence  ?  The  portion  of  fluid  which 
has  been  thus  forcibly  dragged  over  the  molecules 
refuses  to  return  when  the  poker  has  been  removed  from 
the  line  of  dip ;  the  iron,  as  you  see,  has  become  a  per- 
manent magnet.  By  reversing  its  position  and  tapping 
it  again  we  reverse  its  magnetism.  A  thoughtful  and 
competent  teacher  will  know  how  to  place  these  remark- 
able facts  before  his  pupils  in  a  manner  which  will 
excite  their  interest.  By  the  use  of  sensible  images, 
more  or  less  gross,  he  will  first  give  those  whom  he 
teaches  definite  conceptions,  purifying  these  conceptions 
afterwards,  as  the  minds  of  his  pupils  become  more 
capable  of  abstraction.  By  thus  giving  them  a  distinct 
substratum  for  their  reasonings,  he  will  confer  upon  his 
pupils  a  profit  and  a  joy  which  the  mere  exhibition  of 
facts  without  principles,  or  the  appeal  to  the  bodily 
censes  and  the  power  of  memory  alone,  could  never 
inspire. 


ELEMENTARY  MAGNETISM.  367 

As  an  expansion  of  the  note  at  p.  357,  the  following  extract  may 
6nd  a  place  here  : — 

*  It  is  well  known  that  a  voltaic  current  exerts  an  attractive 
force  upoii  a  second  current,  flowing  in  the  same  direction  ;  and 
that  when  the  directions  are  opposed  to  each  other  the  force  exerted 
is  a  repulsive  one.  By  coiling  wires  into  spirals,  Ampere  was 
enabled  to  make  them  produce  all  the  phenomena  of  attraction 
and  repulsion  exhibited  by  magnets,  and  from  this  it  was  but  a 
Btep  to  his  celebrated  theory  of  molecular  currents.  He  supposed 
the  molecules  of  a  magnetic  body  to  be  surrounded  by  such  currents, 
which,  however,  in  the  natural  state  of  the  body  mutually  neutral- 
ised each  other,  on  account  of  their  confused  grouping.  The  act  of 
magnetisation  he  supposed  to  consist  in  setting  these  molecular 
currents  parallel  to  each  other ;  and,  starting  from  this  principle, 
he  reduced  all  the  phenomena  of  magnetism  to  the  mutual  action 
of  electric  currents. 

*  If  we  reflect  upon  the  experiments  recorded  in  the  foregoing 
pages  from  first  to  last,  we  can  hardly  fail  to  be  convinced  that 
diamagnetic  bodies  operated  on  by  magnetic  forces  possess  a  polarity 
"  the  same  in  kind  as,  but  the  reverse  in  direction  of,  that  acquired 
by  magnetic  bodies."  But  if  this  be  the  case,  how  are  we  to 
conceive  the  physical  mechanism  of  this  polarity  ?  According  to 
Coulomb's  and  Poisson's  theory,  the  act  of  magnetisation  consists 
in  the  decomposition  of  a  neutral  magnetic  fluid  ;  the  north  pole  of 
a  magnet,  for  example,  possesses  an  attraction  for  the  south  fluid 
of  a  piece  of  soft  iron  submitted  to  its  influence,  draws  the  said 
fluid  towards  it,  and  with  it  the  material  particles  with  which  the 
fluid  is  associated.  To  account  for  diamagnetic  phenomena  this 
theory  seems  to  fail  altogether ;  according  to  it,  indeed,  the  oft- 
used  phrase,  "  a  north  pole  exciting  a  north  pole,  and  a  south  pole 
a  south  pole,"  involves  a  contradiction.  For  if  the  north  fluid  be 
supposed  to  be  attracted  towards  the  influencing  north  pole,  it  is 
absurd  to  suppose  that  its  presence  there  could  produce  repulsion. 
The  theory  of  Ampfere  is  equally  at  a  loss  to  explain  diamagnetic 
action ;  for  if  we  suppose  the  particles  of  bismuth  surrovmded  by 
molecular  currents,  then,  according  to  all  that  is  known  of  electro- 
dynamic  laws,  these  currents  would  set  themselves  parallel  to,  and 
in  the  same  direction  as,  those  of  the  magnet,  and  hence  attraction, 
and  not  repulsion,  would  be  the  result.  The  fact,  however,  of  this 
not  being  the  case,  proves  that  these  molecular  currents  are  not 
the  mechanism  by  which  diamagnetic  induction  is  effected.  The 
consciousness  of  this,  I  doubt  not,  drove  M.  Weber  to  the  assumption 
that  the  phenomena  of  diamagnetism  are  produced  by  molecular 
currents,  not  directed,  but  actually  excited  in  the  bismuth  by  tho 


368  FRAGMENTS  OF  SCIENCE. 

magnet.  Such  induced  currents  would,  according  to  known  laws, 
have  a  direction  opposed  to  those  of  the  indncin^'^  magnet,  and  hence 
would  produce  the  phenomena  of  repulsion.  To  carry  out  the 
assumption  here  made,  M.  Weber  is  obliged  to  suppose  that  the 
molecules  of  diamagnetic  bodies  are  surrounded  by  channels,  in 
which  the  induced  molecular  currents,  once  excited,  continue  to 
flow  without  resistance.' ' — Diamagnetism  and  Magne-orygtallio 
Actimij  p.  136-7. 

*  In  assuming  these  non-resisting  channels  M.  Weber,  it  must  be 
admitted,  did  not  go  beyond  the  assumptions  of  Ampere. 


XVI.  ^ 

ON  FORGED 

A  SPHERE  of  lead  was  suspended  at  a  height  of  16 
feet  above  the  theatre  floor  of  the  Royal  Institu- 
tion. It  was  liberated,  and  fell  by  gravity.  That  weight 
required  a  second  to  fall  to  the  floor  from  that  eleva- 
tion ;  and  the  instant  before  it  touched  the  floor,  it  had 
a  velocity  of  32  feet  a  second.  That  is  to  say,  if  at  that 
instant  the  earth  were  annihilated,  and  its  attraction 
annulled,  the  weight  would  proceed  through  space  at 
the  uniform  velocity  of  32  feet  a  second. 

If  instead  of  being  pulled  downward  by  gravity,  the 
weight  be  cast  upward  in  opposition  to  gravity,  then, 
to  reach  a  height  of  16  feet  it  must  start  with  a 
velocity  of  32  feet  a  second.  This  velocity  imparted 
to  the  weight  by  the  human  hand,  or  by  any  other 
mechanical  means,  would  carry  it  to  the  precise  height 
from  which  we  saw  it  fall. 

Now  the  lifting  of  the  weight  may  be  regarded  as 
so  much  mechanical  work  performed.  By  means  of  a 
ladder  placed  against  the  wall,  the  weight  might  be 
carried  up  to  a  height  of  1 6  feet ;  or  it  might  be  drawn 
up  to  this  height  by  means  of  a  string  and  pulley,  or 
it  might  be  suddenly  jerked  up  to  a  height  of  16  feet. 
The  amount  of  work  done  in  all  these  cases,  as  far  as 
the  raising  of  the  weight  is  concerned,  would  be  abso- 
•  A  discourse  delivered  in  the  Royal  Institution,  Juno  6, 1862. 


370  FRAGMENTS  OF  SCIENCE. 

lutely  the  same.  The  work  done  at  one  and  the  same 
place,  and  neglecting  the  small  change  of  gravity  with 
the  height,  depends  solely  upon  two  things ;  on  the 
quantity  of  matter  lifted,  and  on  the  height  to  which 
it  is  lifted.  If  we  call  the  quantity  or  mass  of  matter  m, 
and  the  height  through  which  it  is  lifted  ^,  then  the 
product  of  m  into  A,,  or  m  A,  expresses,  or  is  propor- 
tional to,  the  amount  of  work  done. 

Supposing,  instead  of  imparting  a  velocity  of  32  feet 
a  second  we  impart  at  starting  twice  this  velocity.  To 
what  height  will  the  weight  rise  ?  You  might  be  dis- 
posed to  answer,  '  To  twice  the  height ; '  but  this  would 
be  quite  incorrect.  Instead  of  twice  16,  or  32  feet,  it 
would  reach  a  height  of  four  times  1 6,  or  64  feet.  So 
also,  if  we  treble  the  starting  velocity,  the  weight  would 
reach  nine  times  the  height ;  if  we  quadruple  the  speed 
at  starting,  we  attain  sixteen  times  the  height.  Thus, 
with  a  four- fold  velocity  of  128  feet  a  second  at  start- 
ing, the  weight  would  attain  an  elevation  of  256  feet. 
With  a  seven-fold  velocity  at  starting,  the  weight  would 
rise  to  49  times  the  height,  or  to  an  elevation  of  784 
feet. 

Now  the  work  done — or,  as  it  is  sometimes  called, 
the  mechanical  effect — other  things  being  constant,  is, 
as  before  explained,  proportional  to  the  height,  and 
as  a  double  velocity  gives  four  times  the  height,  a  treble 
velocity  nine  times  the  height,  and  so  on,  it  is  perfectly 
plain  that  the  mechanical  efifect  increases  as  the  square 
of  the  velocity.  If  the  mass  of  the  body  be  represented 
by  the  letter  m,  and  its  velocity  by  v,  the  mechanical 
eifect  would  be  proportional  to  or  represented  by  m  v^ 
In  the  case  considered,  I  have  supposed  the  weight  to  be 
cast  upward,  being  opposed  in  its  flight  by  the  resist- 
ance of  gravity ;  but  the  same  holds  true  if  the  pro- 
jectile be  sent  into  water,  mud,  earth,  timber,  or  other 


FORCE.  371 

resisting  material.  If,  for  example,  we  double  the 
velocity  of  a  cannon-ball,  we  quadruple  its  mechanical 
effect.  Hence  the  importance  of  augmenting  the  velo- 
city of  a  projectile,  and  hence  the  philosophy  of  Sir 
William  Armstrong  in  using  a  large  charge  of  powdei 
in  his  recent  striking  experiments. 

The  measure  then  of  mechanical  effect  is  the  mass 
of  the  body  multiplied  by  the  square  of  its  velocity. 

Now  in  firing  a  ball  against  a  target  the  projectile, 
after  collision,  is  often  found  hot.  Mr.  Fairbairn 
informs  me  that  in  the  experiments  at  Shoeburyness  it 
is  a  common  thing  to  see  a  flash,  even  in  broad  day- 
light, when  the  ball  strikes  the  target.  And  if  our  lead 
weight  be  examined  after  it  has  fallen  from  a  height 
it  is  also  found  heated.  Now  here  experiment  and 
reasoning  lead  us  to  the  remarkable  law  that,  like 
the  meclianical  effect,  the  amount  of  heat  generated  is 
proportional  to  the  product  of  the  mass  into  the  square 
of  the  velocity.  Double  your  mass,  other  things  being 
equal,  and  you  double  your  amount  of  heat ;  double 
your  velocity,  other  things  remaining  equal,  and  you 
quadruple  your  amount  of  heat.  Here  then  we  have 
common  mechanical  motion  destroyed  and  heat  pro- 
duced. When  a  violin  bow  is  drawn  across  a  string, 
the  sound  produced  is  due  to  motion  imparted  to  the 
air,  and  to  produce  that  motion  muscular  force  has 
been  expended.  We  may  here  correcrtly  say,  that  the 
mechanical  force  of  the  arm  is  converted  into  music. 
In  a  similar  way  we  say  that  the  arrested  motion  of 
our  descending  weight,  or  of  the  cannon-ball,  is  con- 
verted into  heat.  The  mode  of  motion  changes,  but 
motion  still  continues ;  the  motion  of  the  mass  is  con- 
verted into  a  motion  of  the  atoms  of  the  mass ;  and 
these  small  motions,  communicated  to  the  nerves,  pro- 
duce the  sensation  we  call  heat. 


372  FRA.GMENTS  OF  SCIENCE. 

"We  know  the  amount  of  heat  which  a  given  amount 
of  mechanical  force  can  develope.  Our  lead  ball,  for 
example,  in  falling  to  the  earth  generated  a  quantity  of 
heat  sufficient  to  raise  its  own  temperature  three-fifths 
of  a  Fahrenheit  degree.  It  reached  the  earth  with  a 
velocity  of  32  feet  a  second,  and  forty  times  this  velo- 
city would  be  small  for  a  rifle  bullet ;  multiplying  |-ths 
by  the  square  of  40,  we  find  that  the  amount  of  heat 
developed  by  collision  with  the  target  would,  if  wholly 
concentrated  in  the  lead,  raise  its  temperature  960 
degrees.  This  would  be  more  than  sufficient  to  fuse 
the  lead.  In  reality,  however,  the  heat  developed  is 
divided  between  the  lead  and  the  body  against  which 
it  strikes ;  nevertheless,  it  would  be  worth  while  to  pay 
attention  to  this  point,  and  to  ascertain  whether  rifle 
bullets  do  not,  under  some  circumstances,  show  signs 
of  fusion.^ 

From  the  motion  of  sensible  masses,  by  gravity  and 
other  means,  we  now  pass  to  the  motion  of  atoms  towards 
each  other  by  chemical  affinity.  A  collodion  balloon 
filled  with  a  mixture  of  chlorine  and  hydrogen  being 
hung  in  the  focus  of  a  parabolic  mirror,  in  the  focus 
of  a  second  mirror  20  feet  distant  a  strong  electric 
light  was  suddenly  generated  ;  the  instant  the  concen- 
trated light  fell  upon  the  balloon,  the  gases  within  it 
exploded,  hydrochloric  acid  being  the  result.  Here  the 
atoms  virtually  fell  together,  the  amount  of  heat  pro- 
duced showing  the  enormous  force  of  the  collision. 
The  burning  of  charcoal  in  oxygen  is  an  old  experi- 
ment, but  it  has  now  a  significance  beyond  what  it 
used  to  have ;  we  now  regard  the  act  of  combination 
on  the  part  of  the  atoms  of  oxygen  and  coal  as  we  re- 

'  Eight  years  subsequently  this  surmise  was  proved  correct. 
In  the  Franco-German  War  signs  of  fusion  were  observed  in  tLt? 
case  of  bullets  impinging  on  bonesi. 


FORCE.  373 

gard  the  clashing  of  a  falling  weight  against  the 
earth.  The  heat  produced  in  both  cases  is  referable  to 
a  common  cause.  A  diamond,  which  burns  in  oxygen 
as  a  star  of  white  light,  glows  and  burns  in  consequence 
of  the  falling  of  the  atoms  of  oxygen  against  it.  And 
could  we  measure  the  velocity  of  the  atoms  when  they 
clash,  and  could  we  find  their  number  and  weights,  mul- 
tiplying the  weight  of  each  atom  by  the  square  of  its 
velocity,  and  adding  all  together,  we  should  get  a 
number  representing  the  exact  amount  of  lieat  de- 
veloped by  the  union  of  the  ox5^gen  and  carbon. 

Thus  far  we  have  regarded  the  heat  developed  by 
the  clashing  of  sensible  masses  and  of  atoms.  Work  is 
expended  in  giving  motion  to  these  atoms  or  masses, 
and  heat,  is  developed.  But  we  reverse  this  process 
daily,  and  by  the  expenditure  of  heat  execute  work. 
We  can  raise  a  weight  by  heat ;  and  in  this  agent  we 
possess  an  enormous  store  of  mechanical  power.  A 
pound  of  coal  produces  by  its  combination  with  oxygen 
an  amount  of  heat  which,  if  mechanically  applied, 
would  suffice  to  raise  a  weight  of  100  lbs.  to  a  height  of 
20  miles  above  the  earth's  surface.  Conversely,  100  lbs. 
falling  from  a  height  of  20  miles,  and  striking  against 
the  earth,  would  generate  an  amount  of  heat  equal  to 
that  developed  by  the  combustion  of  a  pound  of  coal. 
Wherever  work  is  done  by  heat,  heat  disappears.  A 
gun  which  fires  a  ball  is  less  heated  than  one  which  fires 
blank  cartridge.  The  quantity  of  heat  communicated 
to  the  boiler  of  a  working  steam-engine  is  greater  than 
that  which  could  be  obtained  from  the  re-condensation 
of  the  steam,  after  it  had  done  its  work ;  and  the 
amount  of  work  performed  is  the  exact  equivalent  of 
the  amount  of  heat  lost.  Mr.  Smyth  informed  us  in 
his  interesting  discourse,  that  we  dig  annually  84 
millions  of  tons  of  coal  from  our  pits.     The  amount  of 


374  FRAGMENTS  OF  SCIENCE. 

mechanical  force  represented  by  this  quantity  of  coal 
seems  perfectly  fabulous.  The  combustion  of  a  single 
pound  of  coal,  supposing  it  to  take  place  in  a  minute, 
would  be  equivalent  to  the  work  of  300  horses ;  and  if 
we  suppose  108  millions  of  horses  working  day  and 
night  with  unimpaired  strength,  for  a  year,  their 
united  energies  would  enable  them  to  perform  an 
amount  of  work  just  equivalent  to  that  which  the 
annual  produce  of  our  coal-fields  would  be  able  to 
accomplish. 

Comparing  with  ordinary  gravity  the  force  with 
which  oxygen  and  carbon  unite  together,  chemical 
affinity  seems  almost  infinite.  But  let  us  give  gravity 
fair  play  by  peimitting  it  to  act  throughout  its  entire 
range.  Place  a  bo  iy  at  such  a  distance  from  the  earth 
that  the  attraction  of  our  planet  is  barely  sensible,  and 
let  it  fall  to  the  earth  from  this  di.stance.  It  would 
reach  the  earth  with  a  final  velocity  of  36,747  feet  a 
second ;  and  on  collision  with  the  earth  the  body  would 
generate  about  twice  the  amount  of  heat  generated  by 
the  combustion  of  an  equal  weight  of  coal.  We  have 
stated  that  by  falling  through  a  space  of  1 6  feet  our  lead 
bullet  would  be  heated  three-fifths  of  a  degree  ;  but  a 
body  falling  fr  jm  an  infinite  distance  has  already  used 
up  1,299,999  parts  out  of  1,300,000  of  the  earth's 
pulling  power,  when  it  has  arrived  within  16  feet  of  the 
surface  ;  on  this  space  only  1^00,000^^^  ^^  ^^®  whole  force 
is  exerted. 

Let  us  now  turn  our  thoughts  for  a  moment  from 
the  earth  to  the  sun.  The  researches  of  Sir  John 
Herschel  and  M.  Pouillet  have  informed  us  of  the 
annual  expenditure  of  the  sun  as  regards  heat ;  and  by 
an  easy  calculation  we  ascertain  the  precise  amount  of 
the  expenditure  which  falls  to  the  share  of  our  planet. 
Out  of  2300  million  parts  of  light  and  heat  the  earth 


FORCE.  375 

receives  one.  The  whole  heat  emitted  by  the  sun  in  a 
minute  would  be  competent  to  boil  12,000  millions  of 
cubic  miles  of  ice-cold  water.  How  is  this  enormous 
loss  made  good — whence  is  the  sun's  heat  derived,  and 
by  what  means  is  it  maintained  ?  No  combustion — no 
chemical  affinity  with  which  we  are  acquainted,  would 
be  competent  to  produce  the  temperature  of  the  sun's 
surface.  Besides,  were  the  sun  a  burning  body  merely, 
its  light  and  heat  would  speedily  come  to  an  end. 
Supposing  it  to  be  a  solid  globe  of  coal,  its  combustion 
would  only  cover  4600  years  of  expenditure.  In  this 
short  time  it  would  burn  itself  out.  What  agency 
then  can  produce  the  temperature  and  maintain  the 
outlay  ?  We  have  already  regarded  the  case  of  a  body 
falling  from  a  great  distance  towards  the  earth,  and 
found  that  the  heat  generated  by  its  collision  would  be 
twice  that  produced  by  the  combustion  of  an  equal 
weight  of  coal.  How  much  greater  must  be  the  heat 
developed  by  a  body  falling  against  the  sun!  -The 
maximum  velocity  with  which  a  body  can  strike  the 
earth  is  about  7  miles  in  a  second ;  the  maximum  velo- 
city with  which  it  can  strike  the  sun  is  390  miles  in  a 
second.  And  as  the  heat  developed  by  the  collision  is 
proportional  to  the  square  of  the  velocity  destroyed,  an 
asteroid  falling  into  the  sun  with  the  above  velocity 
would  generate  about  10,000  times  the  quantity  of  heat 
produced  by  the  combustion  of  an  asteroid  of  coal  of 
the  same  weight. 

Have  we  any  reason  to  believe  that  such  bodies 
exist  in  space,  and  that  they  may  be  raining  down  upon 
the  sun  ?  The  meteorites  flashing  through  the  air  are 
small  planetary  bodies,  drawn  by  the  earth's  attraction. 
They  enter  our  atmosphere  with  planetary  velocity, 
and  by  friction  against  the  air  they  are  raised  to  incan- 
descence and  caused  to  emit  light  and  heat.  At  certain 
25 


376  FRAaMENTS  OF  SCIENCE. 

seasons  of  the  year  they  shower  down  upon  us  in  great 
numbers.  In  Boston  240,000  of  them  were  observed 
in  nine  hours.  There  is  no  reason  to  suppose  that  the 
planetary  system  is  limited  to  '  vast  masses  of  enormous 
weighty'  there  is,  on  the  contrary,  reason  to  believe 
that  space  is  stocked  with  smaller  masses,  which  obey 
the  same  laws  as  the  larger  ones.  That  lenticular 
envelope  which  surrounds  the  sun,  and  which  is  known 
to  astronomers  as  the  Zodiacal  light,  is  probably  a  crowd 
of  meteors;  and  moving  as  they  do  in  a  resisting 
medium,  they  must  continually  approach  the  sun. 
Falling  into  it,  they  would  produce  enormous  heat,  and 
this  would  constitute  a  source  from  which  the  annual 
loss  of  heat  might  be  made  good.  The  sun,  according 
to  this  hypothesis,  would  continually  grow  larger  ;  but 
how  much  larger  ?  Were  our  moon  to  fall  into  the 
sun,  it  would  develope  an  amount  of  heat  sufficient  to 
cover  one  or  two  years'  loss ;  and  were  our  earth  to  fall 
into  -the  sun  a  century's  loss  would  be  made  good.  Still, 
our  moon  and  our  earth,  if  distributed  over  the  surface 
of  the  sun,  would  utterly  vanish  from  perception.  In- 
deed, the  quantity  of  matter  competent  to  produce  the 
required  effect  would,  during  the  range  of  history,  cause 
no  appreciable  augmentation  in  the  sun's  magnitude. 
The  augmentation  of  the  sun's  attractive  force  would 
be  more  sensible.  However  this  hypothesis  may  fare 
as  a  representant  of  what  is  going  on  in  nature,  it  cer- 
tainly shows  how  a  sun  might  be  formed  and  main- 
tained on  known  thermo-dynamic  principles. 

Our  earth  moves  in  its  orbit  with  a  velocity  of 
68,040  miles  an  hour.  Were  this  motion  stopped,  an 
amount  of  heat  would  be  developed  sufficient  to  raise 
the  temperature  of  a  globe  of  lead  of  the  same  size  as 
the  earth  384,000  degrees  of  the  centigrade  therm o- 


FORCE.  377 

meter.  It  has  been  prophesied  that  *  the  elements 
shall  melt  with  fervent  heat.'  The  earth's  own  motion 
embraces  the  conditions  of  fulfilment ;  stop  that  motion, 
and  the  greater  part,  if  not  the  whole,  of  our  planet 
would  be  reduced  to  vapour.  If  the  earth  fell  into  the 
sun,  the  amount  of  heat  developed  by  the  shock  would 
be  e([ual  to  that  developed  by  the  combustion  of  a  mass 
of  solid  coal  6435  times  the  earth  in  size. 

There  is  one  other  consideration  connected  with  the 
permanence  of  our  present  terrestrial  conditions,  which 
is  well  worthy  of  our  attention.  Standing  upon  one  of 
the  London  bridges,  we  observe  the  current  of  the 
Thames  reversed,  and  the  water  poured  upward  twice 
a-day.  The  water  thus  moved  rubs  against  the  river's 
bed,  and  heat  is  the  consequence  of  this  friction. 
The  heat  thus  generated  is  in  part  radiated  into 
space  and  lost,  as  far  as  the  earth  is  concerned.  What 
supplies  this  incessant  loss  ?  The  earth's  rotation.  Let 
us  look  a  little  more  closely  at  the  matter.  Imagine 
the  moon  fixed,  and  the  earth  turning  like  a  wheel 
from  west  to  east  in  its  diurnal  rotation.  Suppose  a 
high  mountain  on  the  earth's  surface  approaching  the 
earth's  meridian ;  that  mountain  is,  as  it  were,  laid 
hold  of  by  the  moon ;  it  forms  a  kind  of  handle  by 
which  the  earth  is  pulled  more  quickly  round.  But 
when  the  meridian  is  passed  the  pull  of  the  moon  on 
the  mountain  would  be  in  the  opposite  direction,  it 
would  tend  to  diminish  the  velocity  of  rotation  as  much 
as  it  previously  augmented  it ;  thus  the  action  of  all 
fixed  bodies  on  the  earth's  surface  is  neutralised.  But 
suppose  the  mountain  to  lie  always  to  the  east  of  the 
moon's  meridian,  the  pull  then  would  be  always  exerted 
against  the  earth's  rotation,  the  velocity  of  which 
would  be  diminished  in  a  degree  corresponding  to  the 
strength  of  the  pull.     The  tidal  wave  occupies  this 


378  FRAaMENTS  OF  SCIENCE. 

fosition — it  lies  always  to  the  east  of  the  moon's 
meridian.  The  waters  of  the  ocean  are  in  part  dragged 
a.s  a  brake  along  the  surface  of  the  earth ;  and  as  a 
brake  they  must  diminish  the  velocity  of  the  earth's 
rotation.^  Supposing  then  that  we  turn  a  mill  by  the 
action  of  the  tide,  and  produce  heat  by  the  friction 
of  the  millstones ;  that  heat  has  an  origin  totally  dif- 
ferent from  the  heat  produced  by  another  mill  which 
is  turned  by  a  mountain  stream.  The  former  is  pro- 
duced at  the  expense  of  the  earth's  rotation,  the  latter 
at  the  expense  of  the  sun's  radiation. 

The  sun,  by  the  act  of  vaporisation,  lifts  mechani- 
cally all  the  moisture  of  our  air,  wliich  when  it  con- 
denses falls  in  the  form  of  rain,  and  when  it  freezes  falla 
as  snow.  In  this  solid  form  it  is  piled  upon  the  Alpine 
heights,  and  furnishes  materials  for  glaciers.  But  the 
sun  again  interposes,  liberates  the  solidified  liquid,  and 
permits  it  to  roll  by  gravity  to  the  sea.  The  me- 
chanical force  of  eveiy  river  in  the  world  as  it  rolls 
towards  the  ocean,  is  drawn  from  the  heat  of  the  sun. 
No  streamlet  glides  to  a  lower  level  without  having 
been  first  lifted  to  the  elevation  from  which  it  springs 
by  the  power  of  the  sun.  The  energy  of  winds  is  also 
due  entirely  to  the  same  power. 

But  there  is  still  another  work  which  the  sun  per- 
forms, and  its  connection  with  which  is  not  so  obvious. 
Trees  and  vegetables  grow  upon  the  earth,  and  when 
burned  they  give  rise  to  heat,  and  hence  to  mechanical 
energy.  Whence  is  this  power  derived  ?  You  see  this 
oxide  of  iron,  produced  by  the  falling  together  of  the 
atoms  of  iron  and  oxygen  ;  you  cannot  see  this  trans- 
parent carbonic  acid  gas,  formed  by  the  falling  togethei 

1  Kant  surmised  an  action  of  this  kind. 


FORCE.  379 

of  carbon  and  oxygen.  The  atoms  thus  in  close  union 
resemble  our  lead  weight  while  resting  on  the  earth ; 
but  we  can  wind  up  the  weight  and  prepare  it  for 
another  fall,  and  so  these  atoms  can  be  wound  up  and 
thus  enabled  to  repeat  the  process  of  combination.  In 
the  building  of  plants  carbonic  acid  is  the  material 
from  which  the  carbon  of  the  plant  is  derived;  and 
the  solar  beam  is  the  agent  which  tears  the  atoms 
asunder,  setting  the  oxygen  free,  and  allowing  the 
carbon  to  aggregate  in  woody  fibre.  Let  the  solar  rays 
fall  upon  a  surface  of  sand ;  the  sand  is  heated,  and 
finally  radiates  away  as  much  heat  as  it  receives ;  let 
the  same  beams  fall  upon  a  forest,  the  quantity  of  heat 
given  back  is  less  than  the  forest  receives ;  for  the 
energy  of  a  portion  of  the  sunbeams  is  invested  in 
building  the  trees.  Without  the  sun  the  reduction  of 
the  carbonic  acid  cannot  be  effected,  and  an  amount  of 
sunlight  is  consumed  exactly  equivalent  to  the  mole- 
cular work  done.  Thus  trees  are  formed ;  thus  the 
cotton  on  which  Mr.  Bazley  discoursed  last  Friday  is  pro- 
duced. I  ignite  this  cotton,  and  it  flames  ;  the  oxygen 
again  unites  with  the  carbon;  but  an  amount  of  heat 
equal  to  that  produced  by  its  combustion  was  sacrificed 
by  the  sun  to  form  that  bit  of  cotton. 

We  cannot,  however,  stop  at  vegetable  life,  for  it 
is  the  source,  mediate  or  immediate,  of  all  animal 
life.  The  sun  severs  the  carbon  from  its  oxygen  and 
builds  the  vegetable;  the  animal  consumes  the  vege- 
table thus  formed,  a  reunion  of  the  severed  elements 
takes  place,  producing  animal  heat.  The  process  of 
building  a  vegetable  is  one  of  winding  up ;  the  process 
of  building  an  animal  is  one  of  running  down.  The 
warmth  of  our  bodies,  and  every  mechanical  energy 
which  we  exert,  trace  their  lineage  directly  to  the  sun. 


380  FKAGMENTS  OF  SCIENCE. 

The  fight  of  a  pair  of  pugilists,  the  motion  of  an 
army,  or  the  lifting  of  his  own  body  by  an  Alpine 
climber  up  a  mountain  slope,  are  all  cases  of  mechani- 
cal energy  drawn  from  the  sun.  A  man  weighing  150 
pounds  has  64  pounds  of  muscle ;  but  these,  when  dried, 
reduce  themselves  to  15  pounds.  Doing  an  ordinary 
day's  work,  for  eighty  days,  this  mass  of  muscle  would 
be  wholly  oxidised.  Special  organs  which  do  more  work 
would  be  more  quickly  consumed :  the  heart,  for  ex- 
ample, if  entirely  unsustained,  would  be  oxidised  in 
about  a  week.  Take  the  amount  of  heat  due  to  the 
direct  oxidation  of  a  given  weight  of  food ;  less  heat 
is  developed  by  the  oxidation  of  the  same  amount  of 
food  in  the  working  animal  frame,  and  the  missing 
quantity  is  the  equivalent  of  the  mechanical  work  ac- 
complished by  the  muscles. 

I  might  extend  these  considerations ;  the  work, 
indeed,  is  done  to  my  hand — but  I  am  warned  that 
you  have  been  already  kept  too  long.  To  whom  then 
are  we  indebted  for  the  most  striking  generalisations 
of  this  evening's  discourse  ?  They  are  the  work  of 
a  man  of  whom  you  have  scarcely  ever  heard — 
the  published  labours  of  a  Grerman  doctor,  named 
Mayer.  Without  external  stimulus,  and  pursuing  his 
profession  as  town  physician  in  Heilbronn,  this  man 
was  the  first  to  raise  the  conception  of  the  interaction 
of  heat  and  other  natural  forces  to  clearness  in  his 
own  mind.  And  yet  he  is  scarcely  ever  heard  of,  and 
even  to  scientific  men  his  merits  are  but  partially 
known.  Led  by  his  own  beautiful  researches,  and  quite 
independent  of  Mayer,  Mr.  Joule  published  in  1843 
his  first  paper  on  the  '  Mechanical  Value  of  Heat ; ' 
but  in  1842  Mayer  had  actually  calculated  the 
mechanical  equivalent  of  heat  from  data  which  only 
a  man  of  the  rarest  penetration  could  turn  to  account. 


FORCE.  881 

In  1845  he  published  his  memoir  on  *  Organic 
JNIotion,'  and  applied  the  mechanical  theory  of  heat 
in  the  most  fearless  and  precise  manner  to  vital 
processes.  He  also  embraced  the  other  natural  agents 
in  his  chain  of  conservation.  In  1853  Mr.  Water- 
ston  proposed,  independently,  the  meteoric  theory 
of  the  sun's  heat,  and  in  1854  Professor  William 
Thomson  applied  his  admirable  mathematical  powers 
to  the  development  of  the  theory ;  but  six  years  pre- 
viously the  subject  had  been  handled  in  a  masterly 
manner  by  Mayer,  and  all  that  I  have  said  about  it  has 
been  derived  from  him.  When  we  consider  the  cir- 
cumstances of  Mayer's  life,  and  the  period  at  which  he 
wrote,  we  cannot  fail  to  be  struck  with  astonishment  at 
what  he  has  accomplished.  Here  was  a  man  of  genius 
working  in  silence,  animated  solely  by  a  love  of  his 
subject,  and  arriving  at  the  most  important  results  in 
advance  of  those  whose  lives  were  entirely  devoted  to 
Natural  Philosophy.  It  was  the  accident  of  bleeding  a 
feverish  patient  at  Java  in  1840  that  led  Mayer  to 
speculate  on  these  subjects.  He  noticed  that  the  venous 
blood  in  the  tropics  was  of  a  brighter  red  than  in  colder 
latitudes,  and  his  reasoning  on  this  fact  led  him  into 
the  laboratory  of  natural  forces,  where  he  has  worked 
with  such  signal  ability  and  success.  Well,  you  will 
desire  to  know  what  has  become  of  this  man.  His 
mind,  it  is  alleged,  gave  way ;  it  is  said  he  became  in- 
sane, and  he  was  certainly  sent  to  a  lunatic  asylum.  In  a 
biographical  dictionary  of  his  country  it  is  stated  that 
he  died  there,  but  this  is  incorrect.  He  recovered; 
and,  I  believe,  is  at  this  moment  a  cultivator  of 
vineyards  in  Heilbronn. 


382  FRAGMENTS  OF  SCIENCE. 

Jtme  20,  1 862. 

While  preparing  fur  publication  my  last  course  of 
lectures  on  Heat,  I  wished  to  make  myself  acquainted 
with  all  that  Dr.  Mayer  had  done  in  connection  with  this 
subject.  I  accordingly  wrote  to  two  gentlemen  who 
above  all  others  seemed  likely  to  give  me  the  informa- 
tion which  I  needed.^  Both  of  them  are  Grerman?,  and 
both  particularly  distinguished  in  connection  with  the 
Dynamical  Theory  of  Heat.  Each  of  them  kindly  fur- 
nished me  with  the  list  of  Mayer's  publications,  and  one 
of  them  [Clausius]  was  so  friendly  as  to  order  them  from 
a  bookseller,  and  to  send  them  to  me.  This  friend,  m 
his  reply  to  my  first  letter  regarding  Mayer,  stated 
his  belief  that  I  should  not  find  anything  very  impor- 
tant in  Mayer's  writings;  but  before  forwarding  the 
memoirs  to  me  he  read  them  himself.  His  letter 
accompanying  them  contains  the  following  words: — 
'  I  must  here  retract  the  statement  in  my  last  letter, 
that  you  would  not  find  much  matter  of  importance  in 
Mayer's  writings :  I  am  astonished  at  the  multitude  of 
beautiful  and  correct  thoughts  which  they  contain;'  and 
he  goes  on  to  point  out  various  important  subjects,  in 
the  treatment  of  which  Mayer  had  anticipated  other 
eminent  writers.  My  other  friend,  in  whose  own 
publications  the  name  of  Mayer  repeatedly  occurs,  and 
whose  papers  containing  these  references  were  translated 
some  years  ago  by  myself,  was,  on  the  10th  of  last  month, 
unacquainted  with  the  thoughtful  and  beautiful  essay 
of  Mayer's,  entitled  'Beitrage  zur  Dynamik  des  Him- 
mels ,'  and  in  1854,  when  Professor  William  Thomson 
developed  in  so  striking  a  manner  the  meteoric  theory 
of  the  sun's  heat,  he  was  certainly  not  aware  of  the 
existence  of  that  essay,  though  from  a  recent  article  in 

^  Helmholtz  and  Clausixis. 


TORCB.  383 

*Macmillan's  Magazine'  I  infer  that  he  is  now  aware  of 
it.  Mayer's  physiological  writings  have  been  referred 
to  by  physiologists — by  Dr.  Carpenter,  for  example 
— in  terms  of  honouring  recognition.  We  have 
hitherto,  indeed,  obtained  fragmentary  glimpses  of  the 
man,  partly  from  physicists  and  partly  from  physiolo- 
gists; but  his  total  merit  has  never  yet  been  recognised 
as  it  assuredly  would  have  been  had  he  chosen  a  happier 
mode  of  publication.  I  do  not  think  a  greater  disservice 
could  be  done  to  a  man  of  science,  than  to  overstate  his 
claims :  such  overstatement  is  sure  to  recoil  to  the  dis- 
advantage of  him  in  whose  interest  it  is  made.  But 
when  Mayer's  opportunities,  achievements,  and  fate  are 
taken  into  account,  I  do  not  think  that  I  shall  be  deeply 
blamed  for  attempting  to  place  him  in  that  honourable 
position,  which  I  believe  to  be  his  due. 

Here,  however,  are  the  titles  of  Mayer's  papers,  the 
perusal  of  which  will  correct  any  error  of  judgment 
into  which  I  may  have  fallen  regarding  their  author. 
'Bemerkungen  liber  die  Krafte  der  unbelebten  Natur,' 
Liebig's  '  Annalen,'  1842,  Vol.  42,  p.  231  ;  'DieOrgan- 
ische  Bewegung  in  ihrem  Zusammenhange  mit  dem 
Stoffwechsel,'  Heilbronn,  1845;  'Beitrage  zurDynamik 
des  Himmels,'  Heilbronn,  1 848 ;  '  Bemerkungen  uber 
das  Mechanische  Equivalent  der  Warme,'  Heilbronn, 
1851. 


In  memoriam. — Dr.  Julius  Eobert  Mayer  died  at 
Heilbronn  on  March  20, 1878,  aged  63  years.  It  gives 
me  pleasure  to  reflect  that  the  great  position  which  he 
will  for  ever  occupy  in  the  annals  of  science  was  first 
virtually  assigned  to  him  in  the  foregoing  discourse. 
He  was  subsequently  chosen  by  acclamation  a  member  of 
the  French  Academy  of  Sciences  ;  and  he  received  from 


384  FEAGMENTS  OF  SCIENCE. 

fche   Royal    Society   the    Copley   medal — it/S    highest 
reward.* 


November  1878. 

At  the  meeting  of  the  British  Association  at  Glas- 
gow in  1876 — that  is  to  say,  more  than  fourteen  years 
after  its  delivery  and  publication — the  foregoing  lecture 
was  made  the  cloak  for  an  unseemly  personal  attack  by 
Professor  Tait.  The  anger  which  found  this  uncour- 
teous  vent  dates  from  ISGS,'^  when  it  fell  to  my  lot  to 
maintain,  in  opposition  to  him  and  a  more  eminent 
colleague,  the  position  which  in  1862  I  had  assigned  to 
Dr.  Mayer.  In  those  days  Professor  Tait  denied  to 
Mayer  all  originality,  and  he  has  since,  I  regret  to  say, 
never  missed  an  opportunity,  however  small,  of  carping 
at  Mayer's  claims.  The  action  of  the  Academy  of 
Sciences  and  of  the  Royal  Society  summarily  disposes 
of  this  detraction,  to  which  its  object,  during  his  life- 
time, never  vouchsafed  either  remonstrance  or  reply. 

Some  time  ago  Professor  Tait  published  a  volume  of 
lectures  entitled '  Recent  Advances  in  Physical  Science,' 
which  I  have  reason  to  know  has  evoked  an  amount  of 
censure  far  beyond  that  hitherto  publicly  expressed. 
Many  of  the  best  heads  on  the  continent  of  Em-ope 
agree  in  their  rejection  and  condemnation  of  the  historic 
portions  of  this  book.  In  March  last  it  was  subjected 
to  a  brief  but  pungent  critique  by  Du  Bois-Reymond, 
the  celebrated  Perpetual  Secretary  of  the  Academy  of 
Sciences  in  Berlin.  Du  Bois-Reymond's  address  was 
on  'National  Feeling,'  and  his  critique  is  thus  wound 
up: — 'The  author  of  the  "Lectures"  is  not,  perhaps, 

»  See  *  The  Copley  Medalist  for  1871,'  p.  479. 

•  See  *  Philosophical  Magazine '  for  this  and  the  succeeding  years. 


FORCE.  885 

sufficiently  well  acquainted  with  the  history  on  which 
he  professes  to  throw  light,  and  on  the  later  phases  of 
which  he  passes  so  unreserved  (schrofif)  a  judgment. 
He  thus  exposes  himself  to  the  suspicion — which,  un- 
happily, is  not  weakened  by  his  other  writings — that 
the  fiery  Celtic  blood  of  his  country  occasionally  runs 
away  with  him,  converting  him  for  the  time  into  a  scien- 
tific Chauvin.  Scientific  Chauvinism,'  adds  the  learned 
secretary,  'from  which  Grerman  investigators  have 
hitherto  kept  free,  is  more  reprehensible  (gehassig) 
than  political  Chauvinism,  inasmuch  as  self-control 
{aitUiche  Haltung)  is  more  to  be  expected  from  men 
of  science,  than  from  the  politically  excited  mass.* 

In  the  case  before  this  '  expectation '  would,  I  fear, 
be  doomed  to  disappointment.  But  Du  Bois-Reymond 
and  his  countrymen  must  not  accept  the  writings  of 
Professor  Tait  as  representative  of  the  thought  of 
England.  Surely  no  nation  in  the  world  has  more 
effectually  shaken  itself  free  from  scientific  Chauvinism. 
From  the  day  that  Davy,  on  presenting  the  Copley 
medal  to  Arago,  scornfully  brushed  aside  that  spurious 
patriotism  which  would  run  national  bounq-aries  through 
the  free  domain  of  science,  chivalry  towards  foreigners 
has  been  a  guiding  principle  with  the  Eoyal  Society. 

On  the  more  private  amenities  indulged  in  by  Pro- 
fessor Tait,  I  do  not  consider  it  necessary  to  say  a 
word. 

'  Festrede,  delivered  before  the  Academy  of  Sciences  of  Berlin, 
in  celebration  of  the  birthday  of  the  Emperor  and  King,  March  28, 
1878. 


886  FEAGMENTS  OF  SCIENCE. 


xvn. 

CONTRIBUTIONS  TO  MOLECULAR  PHYSICS} 

HAVING-  on  previous  occasions  dwelt  upon  the 
enormous  differences  which  exist  among  gaseous 
bodies  both  as  regards  their  power  of  absorbing  and 
emitting  radiant  heat,  I  have  now  to  consider  the  effect 
of  a  change  of  aggregation.  When  a  gas  is  condensed 
to  a  liquid,  or  a  liquid  congealed  to  a  solid,  the  mole- 
cules coalesce,  and  grapple  with  each  other  by  forces 
which  are  insensible  as  long  as  the  gaseous  state  is 
maintained.  But,  even  in  the  solid  and  liquid  con- 
ditions, the  luminiferous  ether  still  surrounds  the  mole- 
cules:  hence,  if  the  acts  of  radiation  and  absorption 
depend  on  them  individually,  regardless  of  their  state  of 
aggregation,  the  change  from  the  gaseous  to  the  liquid 
state  ought  not  materially  to  affect  the  radiant  and 
absorbent  power.  If,  on  the  contrary,  the  mutual 
entanglement  of  the  molecules  by  the  force  of  cohesion 
be  of  paramount  influence,  then  we  may  expect  that 
liquids  will  exhibit  a  deportment  towards  radiant  heat 
altogether  different  from  that  of  the  vapours  from  which 
they  are  derived. 

The  first  part  of  an  enquiry  conducted  in  1863-64 
was  devoted  to  an  exhaustive  examination  of  this  ques- 
tion.    Twelve  different  liquids  were  employed,  and  five 

*  A  discourse  delivered  at  the  Royal  Institution,  March  18, 1864 — ■ 
Bapplementiug,  though  of  prior  date,  the  Beds  Lecture  on  Badiation. 


CONTRIBUTIONS  TO  MOLECULAE  PHYSICS.       387 

different  layers  of  each,  varying  in  thickness  from  0*02  of 
an  inch  to  0*27  of  an  inch.  The  liquids  were  enclosed, 
not  in  glass  vessels,  which  would  have  raaterially  modi- 
fied the  incident  heat,  but  between  plates  of  transparent 
rock-salt,  which  only  slightly  afifected  the  radiation. 
The  source  of  heat  throughout  these  comparative  experi- 
ments consisted  of  a  platinum  wire,  raised  to  incandes- 
cence by  an  electric  current  of  unvarying  strength.  The 
quantities  of  radiant  heat  absorbed  and  transmitted  by 
each  of  the  liquids  at  the  respective  thicknesses  were  first 
determined.  The  vapours  of  these  liquids  were  subse- 
quently examined,  the  quantities  of  vapour  employed 
being  rendered  proportional  to  the  quantities  of  liquid 
previously  traversed  by  the  radiant  heat.  The  result 
was  that,  for  heat  from  the  same  source,  the  order  of 
absorption  of  liquids  and  of  their  vapours  proved 
absolutely  the  same.  There  is  no  known  exception  to 
this  law ;  so  that,  to  determine  the  position  of  a  vapour 
as  an  absorber  or  a  radiator,  it  is  only  necessary  to 
determine  the  position  of  its  liquid. 

This  result  proves  that  the  state  of  aggregation,  as 
far  at  all  events  as  the  liquid  stage  is  concerned,  is  of 
altogether  subordinate  moment — a  conclusion  which 
will  probably  prove  to  be  of  cardinal  importance  in 
molecular  physics.  On  one  important  and  contested 
point  it  has  ^  special  bearing.  If  the  position  of  a  liquid 
as  an  absorber  and  radiator  determine  that  of  its  vapour, 
the  position  of  water  fixes  that  of  aqueous  vapour. 
Water  has  been  compared  with  other  liquids  in  a  multi- 
tude of  experiments,  and  it  has  been  found,  both  as  a 
radiant  and  as  an  absorbent,  to  transcend  them  all. 
Thus,  for  example,  a  layer  of  bisulphide  of  carbon  0*02 
of  an  inch  in  thickness  absorbs  6  per  cent.,  and  allows 
94  per  cent,  of  the  radiation  from  the  red-hot  platinum 
spiral  to  pass  through  it ;  benzol  absorbs  43  and  tran&- 


388  FRAGMENTS  OF  SCIENCE. 

mits  57  per  cent,  of  the  same  radiation ;  alcohol  absorbs 
67  and  transmits  33  per  cent.,  and  alcohol,  as  an  absorber 
of  radiant  heat,  stands  at  the  head  of  all  liquids  except 
one.  The  exception  is  water.  A  layer  of  this  sub- 
stance, of  the  thickness  above  given,  absorbs  81  per 
cent.,  and  permits  only  19  per  cent,  of  the  radiation  to 
pass  through  it.  Had  no  single  experiment  ever  been 
made  upon  the  vapour  of  water,  its  vigorous  action  upon 
radiant  heat  might  be  inferred  from  the  deportment  of 
the  liquid. 

The  relation  of  absorption  and  radiation  to  the 
chemical  constitution  of  the  radiating  and  absorbing 
substances  was  next  briefly  considered.  For  the  first 
six  substances  in  the  list  of  liquids  examined,  the  radiant 
and  absorbent  powers  augment  as  the  number  of  atoms 
in  the  compound  molecule  augments.  Thus,  bisulphide 
of  carbon  has  3  atoms,  chloroform  5,  iodide  of  ethyl  8, 
benzol  12,  and  amylene  15  atoms  in  their  respective 
molecules.  The  order  of  their  power  as  radiants  and 
absorbents  is  that  here  indicated,  bisulphide  of  carbon 
being  the  feeblest,  and  amylene  the  strongest  of  the  six. 
Alcohol,  however,  excels  benzol  as  an  absorber,  though 
it  has  but  9  atoms  in  its  molecule ;  but,  on  the  other 
hand,  its  molecule  is  rendered  more  complex  by  the 
introduction  of  a  new  element.  Benzol  contains  car- 
bon and  hydrogen,  while  alcohol  contains  carbon, 
hydrogen  and  oxygen.  Thus,  not  only  does  atomic 
Tnultitude  come  into  play  in  absorption  and  radiation 
— atomic  complexity  must  also  be  taken  into  account. 
I  would  recommend  to  the  particular  attention  of 
chemists  the  molecule  of  water ;  the  deportment  of  this 
substance  towards  radiant  heat  being  perfectly  anoma- 
lous, if  the  chemical  formula  at  present  ascribed  to  it 
be  correct. 

Sir  William  Herschel  made  the  important  discovery 


CONTKIBUTIONS  TO  MOLECULAR  PHYSICS.       389 

that,  beyond  the  limits  of  the  red  end  of  the  solar  spec- 
trum, rays  of  high  heating  power  exist  which  are  incom- 
petent to  excite  vision.  The  discovery  is  capable  of 
extension.  Dissolving  iodine  in  the  bisulphide  of  car- 
bon, a  solution  is  obtained  which  entirely  intercepts  the 
light  of  the  most  brilliant  flames,  while  to  the  ultra^-red 
rays  of  such  flames  the  same  iodine  is  found  to  be  per- 
fectly diathermic.  The  transparent  bisulphide,  which 
is  highly  pervious  to  invisible  heat,  exercises  on  it 
the  same  absorption  as  the  perfectly  opaque  solution. 
A  hollow  prism  filled  with  the  opaque  liquid  being  placed 
in  the  path  of  the  beam  from  an  electric  lamp,  the 
light-spectrum  is  completely  intercepted,  but  the  heat- 
spectrum  may  be  received  upon  a  screen  and  there 
examined.  P^alling  upon  a  thermo-electric  pile,  its  in- 
visible presence  is  shown  by  the  prompt  deflection  of 
even  a  coarse  galvanometer. 

What,  then,  is  the  physical  meaning  of  opacity  and 
transparency  as  regards  light  and  radiant  heat  ?  The 
visible  rays  of  the  spectrum  differ  from  the  invisible 
ones  simply  in  period.  The  sensation  of  light  is 
excited  by  waves  of  ether  shorter  and  more  quickly 
recurrent  than  the  non-visual  waves  which  fall  beyond 
the  extreme  red.  But  why  should  iodine  stop  the 
former  and  allow  the  latter  to  pass  ?  The  answer  to 
this  question  no  doubt  is,  that  the  intercepted  waves 
are  those  whose  periods  of  recurrence  coincide  with  the 
periods  of  oscillation  possible  to  the  atoms  of  the  dis- 
solved iodine.  The  elastic  forces  which  keep  these 
atoms  apart  compel  them  to  vibrate  in  definite  periods, 
and,  when  these  periods  synchronise  with  those  of  the 
ethereal  waves,  the  latter  are  absorbed.  Briefly  de- 
fined, then,  ti*ansparency  in  liquids,  as  well  as  in  gases, 
is  synonymous  with  discord,  while  opacity  is  synony- 
mous with  accord^  between  the  periods  of  the  wav^g  pf 


390  FKAGMENTS  OF  SCIENCE, 

ether  and  those  of  the  molecules  on  which  they  im- 
pinge. 

According  to  this  view  transparent  and  colourless 
substances  owe  their  transparency  to  the  dissonance 
existing  between  the  oscillating  periods  of  their  atoms 
and  those  of  the  waves  of  the  whole  visible  spec- 
trum. From  the  prevalence  of  transparency  in  com- 
pound bodies,  the  general  discord  of  the  vibrating 
periods  of  their  atoms  with  the  light-giving  waves 
of  the  spectrum,  may  be  inferred  ;  while  their  synchro- 
nism with  the  ultra-red  periods  is  to  be  inferred  from 
their  opacity  to  the  ultra-red  rays.  Water  illustrates 
this  in  a  most  striking  manner.  It  is  highly  trans- 
parent to  the  luminous  rays,  which  proves  that  its 
atoms  do  not  readily  oscillate  in  the  periods  which 
excite  vision.  It  is  highly  opaque  to  the  ultra-red 
undulations,  which  proves  the  synchronism  of  its  vibra- 
ting periods  with  those  of  the  longer  waves. 

If,  then,  to  the  radiation  from  any  source  water 
shows  itself  eminently  or  perfectly  opaque,  we  may 
infer  that  the  atoms  whence  the  radiation  emanates 
oscillate  in  ultra-red  periods.  Let  us  apply  this  test 
to  the  radiation  from  a  flame  of  hydrogen.  This 
flame  consists  mainly  of  incandescent  aqueous  vapour, 
the  temperature  of  which,  as  calculated  by  Bunsen, 
is  3259°  C,  so  that,  if  the  penetrative  power  of 
radiant  heat,  as  generally  supposed,  augment  with  the 
temperature  of  its  source,  we  may  expect  the  radia- 
tion from  this  flame  to  be  copiously  transmitted  by 
water.  While,  however,  a  layer  of  the  bisulphide 
of  carbon  0*07  of  an  inch  in  thickness  transmits  72  per 
cent,  of  the  incident  radiation,  and  while  every  other 
liquid  examined  transmits  more  or  less  of  the  heat,  a 
layer  of  water  of  the  above  thickness  is  entirely  opaque  to 
the  radiation  from  the  hydrogen  flame.  Thus  we  establish 


CONTKIBUTIONS  TO  MOLECULAK  PHYSICS.        391 

accord  between  the  periods  of  the  atoms  of  cold  water 
and  those  of  aqueous  vapour  at  a  temperature  of  3259°  C. 
But  the  periods  of  water  have  already  been  proved  to  be 
ultra  red — whence  those  of  the  hydrogen  flame  must  be 
sensibly  ultra-red  also.  The  absorption  by  dry  air  of 
the  heat  emitted  by  a  platinum  spiral  raised  to  in- 
candescence by  electricity  is  insensible,  while  that  by 
the  ordinary  undried  air  is  6  per  cent.  Substituting 
for  the  platinum  spiral  a  hydrogen  flame,  the  absorp- 
tion by  dry  air  still  remains  insensible,  while  that  of 
the  undried  air  rises  to  20  per  cent,  of  the  entire  radia- 
tion. The  temperature  of  the  hydrogen  flame  is,  as 
stated,  3259°  C. ;  that  of  the  aqueous  vapour  of  the 
air  20°  C.  Suppose,  then,  the  temperature  of  aqueous 
vapour  to  rise  from  20°  C.  to  3259°  C,  we  must  con- 
clude that  the  augmentation  of  temperature  is  applied 
to  an  increase  of  amplitude  or  width  of  swing,  and  not 
to  the  introduction  of  quicker  periods  into  the  radia- 
tion. 

The  part  played  by  aqueous  vapour  in  the  economy 
of  nature  is  far  more  wonderful  than  has  been  hitherto 
supposed.  To  nourish  the  vegetation  of  the  earth  the 
actinic  and  luminous  rays  of  the  sun  must  penetrate 
our  atmosphere ;  and  to  such  rays  aqueous  vapour  is 
eminently  transparent.  The  violet  and  the  ultra-violet 
rays  pass  through  it  with  freedom.  To  protect  vegeta- 
tion from  destructive  chills  the  terrestrial  rays  must  be 
checked  in  their  transit  towards  stellar  space ;  and  this 
is  accomplished  by  the  aqueous  vapour  diffused  through 
the  air.  This  substance  is  the .  great  moderator  of  the 
earth's  temperature,  bringing  its  extremes  into  prox- 
imity, and  obviating  contrasts  between  day  and  night 
which  would  render  life  insupportable.  But  we  can 
advance  beyond  this  general  statement,  now  that  we 
know  the  radiation  from  aqueous  vapour  is  intercepted, 

36 


392  FEAGMENTS  OF  SCIENCE. 

in  a  special  degree,  by  water,  and,  reciprocally,  the  radi- 
ation from  water  by  aqueous  vapour ;  for  it  follows  from 
this  that  the  very  act  of  nocturnal  refrigeration  which 
produces  the  condensation  of  aqueous  vapour  at  the 
surface  of  the  earth — giving,  as  it  were,  a  varnish  of 
water  to  that  surface — imparts  to  terrestrial  radiation 
that  particular  character  which  disqualifies  it  from 
passing  through  the  earth's  atmosphere  and  losing  itself 
in  space. 

And  here  we  come  to  a  question  in  molecular  physics 
which  at  the  present  moment  occupies  attention.  By 
allowing  the  violet  and  ultra-violet  rays  of  the  spectrum 
to  fall  upon  sulphate  of  quinine  and  other  substances 
Professor  Stokes  has  changed  the  periods  of  those  rays. 
Attempts  have  been  made  to  produce  a  similar  result  at 
the  other  end  of  the  spectrum — to  convert  the  ultra-red 
periods  into  periods  competent  to  excite  vision — but 
hitherto  without  success.  Such  a  change  of  period,  I 
agi*ee  with  Dr.  Miller  in  believing,  occurs  when  the  lime- 
light is  produced  by  an  oxy-hydrogen  flame.  In  this 
common  experiment  there  is  an  actual  breaking  up  of  long 
periods  into  short  ones — a  true  rendering  of  unvisual 
periods  visual.  The  change  of  refrangibility  here  effected 
differs  from  that  of  Professor  Stokes ;  firstly,  by  its 
being  in  the  opposite  direction — that  is,  from  a  lower 
refrangibility  to  a  higher ;  and,  secondly,  in  the  circum- 
stance that  the  lime  is  heated  by  the  collision  of  the  mole- 
cules of  aqueous  vapour,  before  their  heat  has  assumed 
the  radiant  form.  But  it  cannot  be  doubted  that  the 
same  effect  would  be  produced  by  radiant  heat  of  the  same 
periods,  provided  the  motion  of  the  aether  could  be  ren- 
dered sufficiently  intense.^  The  effect  in  principle  is  the 
same,   whether  we  consider  the   lime  to  be  struck  by 

^  This  was  soon  afterwards  accomplished.    See  pp.  48,  49. 


CONTRIBUTIONS  TO  MOLECULAR  PHYSICS.        393 

a  particle  of  aqueous  vapour  oscillating  at  a  certain  rate, 
or  by  a  particle  of  ether  oscillating  at  the  same  rate. 

By  plunging  a  platinum  wire  into  a  hydrogen  flame 
we  cause  it  to  glow,  and  thus  introduce  shorter  periods 
into  the  radiation.  These,  as  already  stated,  are  in 
discord  with  the  atomic  vibrations  of  water ;  hence  we 
m?i,j  infer  that  the  transmission  through  water  will  be 
rendered  more  copious  by  the  introduction  of  the  wire 
into  the  flame.  Experiment  proves  this  conclusion  to  be 
true.  Water,  from  being  opaque,  opens  a  passage  to  6 
per  cent,  of  the  radiation  from  the  spiral.  A  thin  plate 
of  colourless  glass,  moreover,  transmits  58  per  cent. 
of  the  radiation  from  the  hydrogen  flame ;  but  when 
the  flame  and  spiral  are  employed,  78  per  cent,  of  the 
heat  is  transmitted. 

For  an  alcohol  flame  Knoblauch  and  Melloni  found 
glass  to  be  less  transparent  than  for  the  same  flame 
with  a  platinum  spiral  immersed  in  it ;  but  Melloni 
afterwards  showed  that  the  result  was  not  general 
— that  black  glass  and  black  mica  were  decidedly 
more  diathermic  to  the  radiation  from  the  pure 
alcohol  flame.  Melloni  did  not  explain  this,  but  the 
reason  is  now  obvious.  The  mica  and  glass  owe 
their  blackness  to  the  carbon  diffused  through  them. 
This  carbon,  as  flrst  proved  by  Melloni,  is  in  some 
measure  transparent  to  the  ultra-red  rays,  and  I  have 
myself  succeeded  in  transmitting  between  40  and  50 
per  cent,  of  the  radiation  from  a  hydrogen  flame 
through  a  layer  of  carbon  which  intercepted  the 
light  of  an  intensely  brilliant  flame.  The  products 
of  combustion  of  alcohol  are  carbonic  acid  and 
aqueous  vapour,  the  heat  of  which  is  almost  wholly 
ultra-red.  For  this  radiation,  then,  the  carbon  is  in  a 
considerable  degree  transparent,  while  for  the  radiation 
from   the   platinum   spiral,  it  is  in  a  great  measure 


394  FKAGMENTS  OF  SCIENCE. 

opaque.  The  platinum  wire,  therefore,  which  aug- 
mented the  radiation  through  the  pure  glass,  augmented 
the  absorption  of  the  black  glass  and  mica. 

No  more  striking  or  instructive  illustration  of  the 
influence  of  coincidence  could  be  adduced  tlian  that 
furnished  by  the  radiation  from  a  carbonic  oxide  flame. 
Here  the  product  of  combustion  is  carbonic  acid ;  and 
on  the  radiation  from  this  flame  even  the  ordinary- 
carbonic  acid  of  the  atmosphere  exerts  a  powerful 
effect.  A  quantity  of  the  gas,  only  one-thirtieth  of  an 
atmosphere  in  density,  contained  in  a  polished  brass 
tube  four  feet  long,  intercepts  50  per  cent,  of  the 
radiation  from  the  carbonic  oxide  flame.  For  the  heat 
emitted  by  lampblack,  olefiant  gas  is  a  far  more 
powerful  absorber  than  carbonic  acid  ;  in  fact,  for  such 
heat,  with  one  exception,  carbonic  acid  is  the  most 
feeble  absorber  to  be  found  among  the  compound  gases. 
Moreover,  for  the  radiation  from  a  hydrogen  flame 
olefiant  gas  possesses  twice  the  absorbent  power  of 
carbonic  acid,  while  for  the  radiation  from  the  carbonic 
oxide  flame,  at  a  common  pressure  of  one  inch  of  mer- 
cury, the  absorption  by  carbonic  acid  is  more  than 
twice  that  of  olefiant  gas.  Thus  we  establish  the 
coincidence  of  period  between  carbonic  acid  at  a  tem- 
perature of  20°  C.  and  carbonic  acid  at  a  temperature 
of  over  3000°  C,  the  peiiods  of  oscillation  of  both  the 
incandescent  and  the  cold  gas  belonging  to  the  ultra- 
red  portion  of  the  spectrum. 

It  will  be  seen  from  the  foregoing  remarks  and 
experiments  how  impossible  it  is  to  determine  the  eff'ect 
of  temperature  pure  and  simple  on  the  transmission  of 
radiant  heat  if  difierent  sources  of  heat  be  employed. 
Throughout  such  an  examination  the  same  oscillating 
atoms  ought  to  be  retained.  This  is  done  by  heating  a 
platinum  spiral  by  an  electric  current,  the  temperature* 


CONTRIBUTIONS  TO  MOLECUT.AR  PHYSICS.       395 

meanwhile  varying  between  the  widest  possible  limits. 
Their  comparative  opacity  to  the  ultra-red  rays  shows  the 
general  accord  of  the  oscillating  periods  of  the  vapours  re- 
ferred to  at  the  commencement  of  this  lecture  with  those 
of  the  ultra-red  undulations.  Hence,  by  gradually  heat- 
ing a  platinum  wire  from  darkness  up  to  whiteness,  we 
ought  gradually  to  augment  the  discord  between  it  and 
these  vapours,  and  thus  augment  the  transmission.  Ex- 
periment entirely  confirms  this  conclusion.  Formic 
ether,  for  example,  absorbs  45  per  cent,  of  the  radiation 
from  a  platinum  spiral  heated  to  barely  visible  redness ; 
32  per  cent,  of  the  radiation  from  the  same  spiral  at  a  red 
heat ;  26  per  cent,  of  the  radiation  from  a  white-hot 
spiral,  and  only  21  per  cent,  when  the  spiral  is  brought 
near  its  point  of  fusion.  Remarkable  cases  of  inversion 
as  to  transparency  also  occur.  For  barely  visible  red- 
ness formic  ether  is  more  opaque  than  sulphuric  ;  for  a 
bright  red  heat  both  are  equally  transparent;  while,  for 
a  white  heat,  and  still  more  for  a  higher  temperature, 
sulphuric  ether  is  more  opaque  than  formic.  This 
result  gives  us  a  clear  view  of  the  relationship  of  the 
two  substances  to  the  luminiferous  ether.  As  we  intro- 
duce waves  of  shorter  period  the  sulphuric  aether 
augments  most  rapidly  in  opacity ;  that  is  to  say,  its 
accord  with  the  shorter  waves  is  greater  than  that  of 
the  formic.  Hence  we  may  infer  that  the  atoms  of 
formic  ether  oscillate,  on  the  whole,  more  slowly  than 
those  of  sulphuric  ether. 

When  the  source  of  heat  is  a  Leslie's  cube  coated 
with  lampblack  and  filled  with  boiling  water,  the 
opacity  of  formic  aether  in  comparison  with  sulphuric 
is  very  decided.  With  this  source  also  the  positions 
of  chloroform  and  iodide  of  methyl  are  inverted. 
For  a  white-hot  spiral,  the  absorption  of  chloroform 
vapour  being  10  per  cent.,  that  of  iodide  of  methyl  ia 


396  FRAOMENTS  OF  SCIENCE. 

1  f) ;  with  the  blackened  cube  as  source,  the  absorption 
by  chloroform  is  22  per  cent.,  while  that  by  the  iodide 
of  methyl  is  only  1 9.  This  inversion  is  not  the  result 
of  temperature  merely;  for  when  a  platinum  wire, 
heated  to  the  temperature  of  boiling  water,  is  em- 
ployed as  a  source,  the  iodide  continues  to  be  the  most 
powerful  absorber.  All  the  experiments  hitherto  made 
go  to  prove  that  from  heated  lampblack  an  emission 
takes  place  which  synchronises  in  an  especial  manner 
with  chloroform.  For  the  cube  at  100°  C,  coated 
with  lampblack,  the  absorption  by  chloroform  is  more 
than  three  times  that  by  bisulphide  of  carbon  ;  for  the 
radiation  from  the  most  luminous  portion  of  a  gas-flame 
the  absorption  by  chloroform  is  also  considerably  in 
excess  of  that  by  bisulphide  of  carbon ;  while,  for  the 
flame  of  a  Bunsen's  burner,  from  which  the  incan- 
descent carbon  particles  are  removed  by  the  free  ad- 
mixture of  air,  the  absorption  by  bisulphide  of  carbon 
is  nearly  twice  that  by  chloroform.  The  removal  of 
the  carbon  particles  more  than  dovhles  the  relative 
transparency  of  the  chloroform.  Testing,  moreover, 
the  radiation  from  various  parts  of  the  same  flame, 
it  was  found  that  for  the  blue  base  of  the  flame  the 
bisulphide  of  carbon  was  most  opaque,  while  for  aU 
other  parts  of  the  flame  the  chloroform  was  most  opaque. 
For  the  radiation  from  a  very  small  gas  flame,  consisting 
of  a  blue  base  and  a  small  white  tip,  the  bisulphide 
was  also  most  opaque,  and  its  opacity  very  decidedly 
exceeded  that  of  the  chloroform  when  the  source  of 
heat  was  the  flame  of  bisulphide  of  carbon.  Com- 
paring the  radiation  from  a  Leslie's  cube  coated  with 
isinglass  with  that  from  a  similar  cube  coated  with 
lampblack,  at  the  common  temperature  of  100°  C,  it 
was  found  that,  out  of  eleven  vapours,  all  but  one  ab- 
sorbed the  radiation  from  the  isinglass  most  powerfully ; 
the  single  exception  was  chloroform. 


CONTRIBUTIONS  TO   MOLECULAR  PHYSICS.       397 

It  is  worthy  of  remark  that  whenever,  through  a 
change  of  source,  the  position  of  a  vapour  as  an  ab- 
sorber of  radiant  heat  was  altered,  the  position  of  the 
liquid  from  which  the  vapour  was  derived  underwent  a 
similar  change. 

It  is  still  a  point  of  difference  between  eminent 
investigators  whether  radiant  heat,  up  to  a  temperature 
of  100°  C,  is  monochromatic  or  not.  Some  affirm  this ; 
some  deny  it.  A  long  series  of  experiments  enables 
me  to  state  that  probably  no  two  substances  at  a  tem- 
perature of  100°  C.  emit  heat  of  the  same  quality. 
The  heat  emitted  by  isinglass,  for  example,  is  different 
from  that  emitted  by  lampblack,  and  the  heat  emitted 
by  cloth,  or  paper,  differs  from  both.  It  is  also  a  subject 
of  discussion  whether  rock-salt  is  equally  diathermic  to 
all  kinds  of  calorific  rays ;  the  differences  affirmed  to 
exist  by  some  investigators  being  ascribed  by  others  to 
differences  of  incidence  from  the  various  sources  em- 
ployed. MM.  de  la  Provostaye  and  Desains  maintain 
the  former  view,  Melloni  and  M.  Knoblauch  maintain 
the  latter.  I  tested  this  point  without  changing  any- 
thing but  the  temperature  of  the  source ;  its  size, 
distance,  and  surroundings  remaining  the  same.  The 
experiments  proved  rock-salt  tc  be  coloured  thermally. 
It  is  more  opaque,  for  example,  to  the  radiation  from  a 
barely  visible  spiral  than  to  that  from  a  white-hot  one. 

In  regard  to  the  relation  of  radiation  to  conduction, 
if  we  define  radiation,  internal  as  well  as  external,  as 
the  communication  of  motion  from  the  vibrating  atoms 
to  the  ether,  we  may,  I  think,  by  fair  theoretic  reason- 
ing, reach  the  conclusion  that  the  best  radiators  ought 
to  prove  the  worst  conductors.  A  broad  consideration 
of  the  subject  shows  at  once  the  general  harmony  of  this 
conclusion  with  observed  facts.  Organic  substances 
are  all  excellent  radiators  ;  they  are  also  extremely  bad 


398  FRAGMENTS  OF  SCIENCE. 

conductors.  The  moment  we  pass  from  the  metals 
to  their  compounds  we  pass  from  good  conductors  to 
bad  ones,  and  from  bad  radiators  to  good  ones.  Water, 
among  liquids,  is  probably  the  worst  conductor;  it 
is  the  best  radiator.  Silver,  among  solids,  is  the  best 
conductor;  it  is  the  worst  radiator.  The  excellent 
researches  of  MM.  de  la  Provostaye  and  Desains  fur- 
nish a  striking  illustration  of  what  I  am  inclined  to 
regard  as  a  natural  law — that  those  atoms  which 
transfer  the  greatest  amount  of  motion  to  the  ether, 
or,  in  other  words,  radiate  most  powerfully,  are  the 
least  competent  to  communicate  motion  to  each  other, 
or,  in  other  words,  to  propagate  by  conduction  readily. 


XVIII. 

LIFE  AND  LETTERS   OF  FARADAY. 

1870. 

UNDERTAKEN  and  executed  in  a  reverent  and  lov- 
ing spirit,  the  work  of  Dr.  Bence  Jones  makes 
Faraday  the  virtual  writer  of  his  own  life.  Everybody 
now  knows  the  story  of  the  philosopher's  birth ;  that  his 
father  was  a  smith ;  that  he  was  born  at  Newington  Butts 
in  1791;  that  he  ran  along  the  London  pavements,  a 
bright-eyed  errand  boy,  with  a  load  of  brown  curls  upon 
his  head  and  a  packet  of  newspapers  under  his  arm ;  that 
the  lad's  master  was  a  bookseller  and  bookbinder — a 
kindly  man,  who  became  attached  to  the  little  fellow, 
and  in  due  time  made  him  his  apprentice  without  fee  ; 
that  during  his  apprenticesliip  he  found  his  appetite  for 
knowledge  provoked  and  strengthened  by  the  books  he 
stitched  and  covered.  Thus  he  grew  in  wisdom  and 
stature  to  his  year  of  legal  manhood,  when  he  appears 
in  the  volumes  before  us  as  a  writer  of  letters,  which 
reveal  his  occupation,  acquirements,  and  tone  of  mind. 
His  correspondent  was  Mr.  Abbott,  a  member  of  the 
Society  of  Friends,  who,  with  a  forecast  of  his  corre- 
spondent's greatness,  preserved  his  letters  and  produced 
them  at  the  proper  time. 

In  later  years  Faraday  always  carried  in  his  pocket 
a  blank  card,  on  which  he  jotted  down  in  pencil  his 


400  FRAGMENTS  OF  SCIENCE. 

thoughts  and  memoranda.  He  made  his  notes  in  the 
laboratory,  in  the  theatre,  and  in  the  streets.  This 
distrust  of  his  memory  reveals  itself  in  his  first  letter  to 
Abbot.  To  a  proposition  that  no  new  enquiry  should 
be  started  between  them  before  the  old  one  had  been 
exhaustively  discussed,  Faraday  objects.  'Your  notion,' 
he  says,  '  I  can  hardly  allow,  for  the  following  reason : 
ideas  and  thoughts  spring  up  in  my  mind  which  are 
irrevocably  lost  for  want  of  noting  at  the  time.'  Gentle 
as  he  seemed,  he  wished  to  have  his  own  way,  and  he 
had  it  throughout  his  life.  Differences  of  opinion 
sometimes  arose  between  the  two  friends,  and  then  they 
resolutely  faced  each  other.  'I  accept  your  offer  to 
fight  it  out  with  joy,  and  shall  in  the  battle  of  experi- 
ence cause  not  pain,  but,  I  hope,  pleasure.'  Faraday 
notes  his  own  impetuosity,  and  incessantly  checks  it. 
There  is  at  times  something  almost  mechanical  in  his 
self-restraint.  In  another  nature  it  would  have  hardened 
into  mere  'correctness'  of  conduct;  but  his  overflowing 
affections  prevented  this  in  his  case.  The  habit  of  self- 
control  became  a  second  nature  to  him  at  last,  and  lent 
serenity  to  his  later  years. 

In  October  1812  he  was  engaged  by  a  Mr.  De  la 
Roche  as  a  journeyman  bookbinder ;  but  the  situation 
did  not  suit  him.  His  master  appears  to  have  been  an 
austere  and  passionate  man,  and  Faraday  was  to  the  last 
degree  sensitive.  All  his  life  he  continued  so.  He 
suffered  at  times  from  dejection;  and  a  certain  grim- 
ness,  too,  pervaded  his  moods.  'At  present,'  he  writes 
to  Abbott,  '  I  am  as  serious  as  you  can  be,  and  would 
not  scruple  to  speak  a  truth  to  any  human  being,  what- 
ever repugnance  it  might  give  rise  to.  Being  in  this 
state  of  mind,  I  should  have  refrained  from  writing  to 
you,  did  I  not  conceive  from  the  general  tenor  of  your 
letters  that  your  mind  is,  at  proper  times,  occupied  upon 


FAEADAY.  401 

serious  subjects  to  the  exclusion  of  those  that  are 
frivolous.'  Plainly  he  had  fallen  into  that  stern  Puritan 
mood,  which  not  only  crucifies  the  affections  and  lusts 
of  him  who  harbours  it,  but  is  often  a  cause  of  disturbed 
digestion  to  his  friends. 

About  three  months  after  his  engagement  with 
De  la  Eoche,  Faraday  quitted  him  and  bookbinding 
together.  He  had  heard  Davy,  copied  his  lectures,  and 
written  to  him,  entreating  to  be  released  from  Trade, 
which  he  hated,  and  enabled  to  pursue  Science.  Davy 
recognised  the  merit  of  his  correspondent,  kept  his  eye 
upon  him,  and,  when  occasion  offered,  drove  to  his  door 
and  sent  in  a  letter,  offering  him  the  post  of  assistant 
in  the  laboratory  of  the  Royal  Institution.  He  was 
engaged  March  1,  1813,  and  on  the  8th  we  find  him 
extracting  the  sugar  from  beet-root.  He  joined  the 
City  Philosophical  Society  which  had  been  founded  by 
Mr.  Tatum  in  1808.  *The  discipline  was  very  sturdy, 
the  remarks  very  plain,  and  the  results  most  valuable.' 
Faraday  derived  great  profit  from  this  little  association. 
In  the  laboratory  he  had  a  discipline  sturdier  still. 
Both  Davy  and  himself  were  at  this  time  frequently  cut 
and  bruised  by  explosions  of  chloride  of  nitrogen.  One 
explosion  was  so  rapid  'as  to  blow  my  hand  open,  tear 
away  a  part  of  one  nail,  and  make  my  fingers  so  sore 
that  I  cannot  use  them  easily.'  In  another  experiment 
'the  tube  and  receiver  were  blown  to  pieces,  I  got  a  cut 
on  the  head,  and  Sir  Humphry  a  bruise  on  his  hand.' 
And  again  speaking  of  the  same  substance,  he  says, 
'when  put  in  the  pump  and  exhausted,  it  stood  for  a 
moment,  and  then  exploded  with  a  fearful  noise.  Both 
Sir  H.  and  I  had  masks  on,  but  I  escaped  this  time  the 
best.  Sir  H.  had  his  face  cut  in  two  places  about  the 
chin,  and  a  violent  blow  on  the  forehead  struck  through 


402  FKAGMENTS  OF  SCIENCE. 

a  considerable  thickness  of  silk  and  leather.'  It  was 
this  same  substance  that  blew  out  the  eye  of  Dulong. 

Over  and  over  again,  even  at  this  early  date,  we  can 
discern  the  quality  which,  compounded  with  his  rare 
intellectual  power,  made  Faraday  a  great  experimental 
philosopher.  This  was  his  desire  ta  see  facts,  and  not 
to  rest  contented  with  the  descriptions  of  them.  He 
frequently  pits  the  eye  against  the  ear,  and  affirms  the 
enormous  superiority  of  the  organ  of  vision.  I^ate  in 
life  I  have  heard  him  say  that  he  could  never  fully 
understand  an  experiment  until  he  had  seen  it.  But 
he  did  not  confine  himself  to  experiment.  He  aspired 
to  be  a  teacher,  and  reflected  and  wrote  upon  the  method 
of  scientific  exposition.  'A  lecturer,'  he  observes, 
'  should  appear  easy  and  collected,  undaunted  and 
unconcerned  : '  still  *  his  whole  behaviour  should  evince 
respect  for  his  audience.'  These  recommendations  were 
fi^fterwards  in  great  part  embodied  by  himself.  I  doubt 
his  'unconcern,'  but  his  fearlessness  was  often  manifested. 
It  used  to  rise  within  him  as  a  wave,  which  carried  both 
him  and  his  audience  along  with  it.  On  rare  occasions 
also,  when  he  felt  himself  and  his  subject  hopelessly 
unintelligible,  he  suddenly  evoked  a  certain  recklessness 
of  thought,  and,  without  halting  to  extricate  his  bewil- 
dered followers,  he  would  dash  alone  through  the  jungle 
into  which  he  had  unwittingly  led  them  ;  thus  saving 
them  from  ennui  by  the  exhibition  of  a  vigour  which, 
for  the  time  being,  they  could  neither  share  nor  com- 
prehend. 

In  October  1813  he  quitted  England  with  Sir  Hum- 
phry and  Lady  Davy.  During  his  absence  he  kept  a 
journal,  from  which  copious  and  interesting  extracts  ha.ve 
been  made  by  Dr.  Bence  Jones.  Davy  was  considerate, 
preferring  at  times  to  be  his  own  servant  rather  than  im- 
pose on  Faraday  duties  which  he  disliked.     But  Lady 


FAEADAY,  403 

Davy  was  the  reverse.  She  treated  him  as  an  underling ; 
he  chafed  under  the  treatment,  and  was  often  on  the  point 
of  returning  home.  They  halted  at  Geneva.  De  la  Eive, 
the  elder, had  known  Davy  in  1799,  and,  by  his  writings 
in  the  '  Bibliotlie(|ue  Britannique,'  had  been  the  first  to 
make  the  English  chemist's  labours  known  abroad.  He 
welcomed  Davy  to  his  country  residence  in  1814.  Both 
were  sportsmen,  and  they  often  went  out  shooting 
together.  On  these  occasions  F'aiaday  charged  Davy's 
gun  while  De  la  Rive  charged  his  own.  Once  the 
Grenevese  philosopher  found  himself  by  the  side  of  Fara- 
day, and  in  his  frank  and  genial  way  entered  into  con- 
versation with  the  young  man.  It  was  evident  that  a 
person  possessing  such  a  charm  of  manner  and  such 
high  intelligence  could  be  no  mere  servant.  On  enquiry 
De  la  Eive  was  somewhat  shocked  to  find  that  the  soi- 
disant  domestique  was  really  preparateur  in  the  labo- 
ratory of  the  Royal  Institution ;  and  he  immediately 
proposed  that  P'araday  thenceforth  should  join  the 
masters  instead  of  the  servants  at  their  meals.  To  this 
Davy,  probably  out  of  weak  deference  to  his  wife, 
objected  ;  but  an  arrangement  was  come  to  that  Fara- 
day thenceforward  should  have  his  food  in  his  own  room. 
Rumour  states  that  a  dinner  in  honour  of  Faraday  was 
given  by  De  la  Rive.  This  is  a  delusion  ;  there  was  no 
such  banquet ;  but  Faraday  never  forgot  the  kindness  or 
the  friend  who  saw  his  merit  when  he  was  a  mere  gavQon 
de  laboratoireJ 

•  While  confined  last  autumn  at  Geneva  by  the  effects  of  a  fall 
in  the  Alps,  my  friends,  with  a  kindness  I  can  never  forget,  did  all 
fchat  friendship  could  suggest  to  render  my  captivity  pleasant  to 
me.  M.  de  la  Eive  then  wrote  out  for  me  the  full  account,  of 
which  the  foregoing  is  a  condensed  abstract.  It  was  at  tlie  desire 
of  Dr.  Bence  Jones  that  I  asked  him  to  do  so.  The  rumour  of  a 
banquet  at  Geneva  illustrates  the  tendency  to  substitute  for  the 
youth  of  1814  the  Faraday  of  later  years- 


404  FRAGMENTS  OF  SCIENCE, 

He  returned  in  1815  to  the  Royal  Institution.  Here 
he  helped  Davy  for  years ;  he  worked  also  for  himself, 
and  lectured  frequently  at  the  City  Philosophical  Society. 
He  took  lessons  in  elocution,  happily  without  damage  to 
his  natural  force,  earnestness,  and  grace  of  delivery.  He 
was  never  pledged  to  theory,  and  he  changed  in  opinion 
as  knowledge  advanced.  With  him  life  was  growth. 
In  those  early  lectures  we  hear  him  say,  '  In  knowledge, 
that  man  only  is  to  be  contemned  and  despised  who  is 
not  in  a  state  of  transition.'  And  again  :  '  Nothing  is 
more  difficult  and  requires  more  caution  than  philoso- 
phical deduction,  nor  is  there  anythmg  more  adverse 
to  its  accuracy  than  fixity  of  opinion.'  Not  that  he  was 
wafted  about  by  every  wind  of  doctrine ;  but  that  he 
united  flexibility  with  his  strength.  In  striking  con- 
trast with  this  intellectual  expansiveness  was  his  fixity 
in  religion,  but  this  is  a  subject  which  cannot  be  dis- 
cussed here. 

Of  all  the  letters  published  in  these  volumes  none 
possess  a  greater  charm  than  those  of  Faraday  to  his 
wife.  Here,  as  Dr.  Bence  Jones  truly  remarks,  '  he  laid 
open  all  his  mind  and  the  whole  of  his  character,  and 
what  can  be  made  known  can  scarcely  fail  to  charm  every 
one  by  its  loveliness,  its  truthfulness,  and  its  earnest- 
ness.' Abbott  and  he  sometimes  swerved  into  word- 
play about  love;  but  up  to  1820,  or  thereabouts,  the 
passion  was  potential  merely.  F'araday's  journal  indeed 
contains  entries  which  show  that  he  took  pleasure  in  the 
assertion  of  his  contempt  for  love ;  but  these  very 
entries  became  links  in  his  destiny.  It  was  througli 
them  that  he  became  acquainted  with  one  who  inspired 
him  with  a  feeling  which  only  ended  with  his  life.  His 
biographer  has  given  us  the  means  of  tracing  the  vary- 
ing moods  which  preceded  his  acceptance.  They  reveal 
more  than  the  common  alternations  of  light  and  gloom. ; 


FAliADAY.  405 

at  one  moment  he  wishes  that  his  flesh  might  melt  and 
that  he  might  become  nothing ;  at  another  he  is  in- 
toxicated with  hope.  The  impetuosity  of  his  character 
was  then  un chastened  by  the  discipline  to  which  it  was 
subjected  in  after  years.  The  very  strength  of  his 
passion  proved  for  a  time  a  bar  to  its  advance,  suggest- 
ing, as  it  did,  to  the  conscientious  mind  of  Miss  Barnard, 
doubts  of  her  capability  to  return  it  with  adequate  force. 
But  they  met  again  and  again,  and  at  each  successive 
meeting  he  found  his  heaven  clearer,  until  at  length  he 
was  able  to  say,  '  Not  a  moment's  alloy  of  this  evening's 
happiness  occurred.  Everything  was  delightful  to  the 
last  moment  of  my  stay  with  my  companion,  because 
she  was  so.'  The  turbulence  of  doubt  subsided,  and  a 
calm  and  elevating  confidence  took  its  place.  '  What 
can  I  call  myself,'  he  writes  to  her  in  a  subsequent 
letter,  *  to  convey  most  perfectly  my  affection  and  love 
for  you  ?  Can  I  or  can  truth  say  more  than  that  for 
this  world  I  am  yours  ? '  Assuredly  he  made  his  pro- 
fession good,  and  no  fairer  light  falls  upon  his  character 
than  that  which  reveals  his  relations  to  his  wife.  Never, 
I  believe,  existed  a  manlier,  purer,  steadier  love.  Like 
a  burning  diamond,  it  continued  to  shed,  for  six-and- 
forty  years,  its  white  and  smokeless  glow. 

Faraday  was  married  on  June  12,  1821 ;  and  up  to 
this  date  Davy  appears  throughout  as  his  friend.  Soon 
afterw^ds,  however,  disunion  occurred  between  them, 
which,  while  it  lasted,  must  have  given  Faraday  intense 
pain.  It  is  impossible  to  doubt  the  honesty  of  conviction 
with  which  this  subject  has  been  treated  by  Dr.  Bence 
Jones,  and  there  may  be  facts  known  to  him,  but  not  ap- 
pearing in  these  volumes,  which  justify  his  opinion  that 
Davy  in  those  days  had  become  jealous  of  Faraday.  This, 
which  is  the  prevalent  belief,  is  also  reproduced  in  an 
excellent  article   in   the    March   number  of  'Frasers 


406  FEAGMENTS  OF  SCIENCE. 

Magazine.*  But  the  best  analysis  I  can  make  of  the 
data  fails  to  present  Davy  in  this  light  to  me.  The 
facts,  as  I  regard  them,  are  briefly  these. 

In  1820,  Oersted  of  Copenhagen  made  the  cele- 
brated discovery  which  connects  electricity  with  mag- 
netism, and  immediately  afterwards  the  acute  mind  of 
WoUaston  perceived  that  a  wire  carrying  a  current  ought 
to  rotate  round  its  own  axis  under  the  influence  of  a 
magnetic  pole.  In  1821  he  tried,  but  failed,  to  realise 
this  result  in  the  laboratory  of  the  Royal  Institution. 
Faraday  was  not  present  at  the  moment,  but  he  came 
in  immediately  afterwards  and  heard  the  conversation 
of  Wollaston  and  Davy  about  the  experiment.  He  had 
also  heard  a  rumour  of  a  wager  that  Dr.  Wollaston 
would  eventually  succeed. 

This  was  in  April.  In  the  autumn  of  the  same 
year  Faraday  wrote  a  history  of  electro-magnetism,  and 
repeated  for  himself  the  experiments  which  he  described. 
It  was  while  thus  instructing  himself  that  he  succeeded 
in  causing  a  wire,  carrying  an  electric  current,  to 
rotate  round  a  magnetic  pole.  This  was  not  the  result 
sought  by  Wolla&ton,  but  it  was  closely  related  to  that 
result. 

The  strong  tendency  of  Faraday's  mind  to  look  upon 
the  reciprocal  actions  of  natural  forces  gave  birth  to 
his  greatest  discoveries ;  and  we,  who  know  this,  should 
be  justified  in  concluding  that,  even  had  Wollaston  not 
preceded  him,  the  result  would  have  been  the  same. 
But  in  judging  Davy  we  ought  to  transport  ourselves 
to  his  time,  and  carefully  exclude  from  our  thoughts 
and  feelings  that  noble  subsequent  life,  whirh  would 
render  simply  impossible  the  ascription  to  Faraday  of 
anything  unfair.  It  would  be  unjust  to  Davy  to  put 
our  knowledge  in  the  place  of  his,  or  to  credit  him 
with  data  which  he  could  not  have  possessed.     Rumour 


FAEADAY.  407 

and  fact  had  connected  the  name  of  Wollaston  with 
these  supposed  interactions  between  magnets  and  cur- 
rents. When,  therefore,  Faraday  in  October  published 
his  successful  experiment,  without  any  allusion  to 
Wollaston,  general,  though  really  ungrounded,  criti- 
cism followed.  I  say  ungrounded  because,  firstly, 
Faraday's  experiment  was  not  that  of  Wollaston,  and 
secondly,  Faraday,  before  he  published  it,  had  actually 
called  upon  Wollaston,  and  not  finding  him  at  home, 
did  not  feel  himself  authorised  to  mention  his  name. 

In  December,  Faraday  published  a  second  paper  on 
the  same  subject,  from  which,  through  a  misappre- 
hension, the  name  of  Wollaston  was  also  omitted. 
Warburton  and  others  thereupon  affirmed  that  Wol- 
laston's  ideas  had  been  appropriated  without  acknow- 
ledgment, and  it  is  plain  that  Wollaston  himself, 
though  cautious  in  his  utterance,  was  also  hurt. 
Censure  grew  till  it  became  intolerable.  '  I  hear,' 
writes  Faraday  to  bis  friend  Stodart,  '  every  day  more 
and  more  of  these  sounds,  which,  though  only  whispers 
to  me,  are,  I  suspect,  spoken  aloud  among  scientific 
men.'  He  might  have  written  explanations  and  de- 
fences, but  he  went  straighter  to  the  point.  He  wished 
to  see  the  principals  face  to  face — to  plead  his  cause 
before  them  personally.  There  was  a  certain  vehemence 
in  his  desire  to  do  this.  He  saw  Wollaston,  he  saw 
Davy,  he  saw  Warburton  ;  and  I  am  inclined  to  think 
that  it  was  the  irresistible  candour  and  truth  of 
character  which  these  viva  voce  defences  revealed,  as 
much  as  the  defences  themselves,  that  disarmed  resent- 
ment at  the  time. 

As  regards  Davy,  another  cause  of  dissension  arose 
in  1823.  In  the  spring  of  that  year  Faraday  analysed 
the  hydrate  of  chlorine,  a  substance  once  believed  to  be 
the   element  chlorine,  but  proved  by  Davy  to  be  a 

57 


408  FllAGMENTS  OF  SCIENCE. 

compound  of  that  element  and  water.  The  analysis 
was  looked  over  by  Davy,  who  then  and  there  suggested 
to  Faraday  to  heat  the  hydrate  in  a  closed  glass  tube. 
This  was  done,  the  substance  was  decomposed,  and  one 
of  the  products  of  decomposition  was  proved  by  Faraday 
to  be  chlorine  liquefied  by  its  own  pressure.  On  the 
day  of  its  discovery  he  communicated  this  result  to 
Dr.  Paris.  Davy,  on  being  informed  of  it,  instantly 
liquefied  another  gas  in  the  same  way.  Having  struck 
thus  into  Faraday's  enquiry,  ought  he  not  to  have  left 
the  matter  in  Faraday's  hands  ?  I  think  he  ought. 
But,  considering  his  relation  to  both  Faraday  and  the 
hydrate  of  chlorine,  Davy,  I  submit,  may  be  excused 
for  thinking  differently.  A  father  is  not  always  wise 
enough  to  see  that  his  son  has  ceased  to  be  a  boy,  and 
estrangement  on  this  account  is  not  rare ;  nor  was 
Davy  wise  enough  to  discern  that  Faraday  had  passed 
the  mere  as.^istant  stage,  and  become  a  discoverer.  It 
is  now  hard  to  avoid  magnifying  this  error.  But  had 
Faraday  died  or  ceased  to  work  at  this  time,  or  had  his 
subsequent  life  been  devoted  to  money-getting,  instead 
of  to  research,  would  anybody  now  dream  of  ascribing 
jealousy  to  Davy?  Assuredly  not.  Why  should  he  be 
jealous  ?  His  reputation  at  this  time  was  almost  with- 
out a  parallel :  his  glory  was  without  a  cloud.  He  had 
added  to  his  other  discoveries  that  of  Faraday,  and 
after  having  been  his  teacher  for  seven  years,  his  lan- 
guage to  him  was  this  :  '  It  gives  me  great  pleasure  to 
hear  that  you  are  comfortable  at  the  Eoyal  Institution, 
and  I  trust  that  you  will  not  only  do  something  good 
and  honourable  for  yourself,  but  also  for  science.'  This 
is  not  the  language  of  jealousy,  potential  or  actual. 
But  the  chlorine  business  introduced  irritation  and 
anger,  to  which,  and  not  to  any  ignobler  motive,  Davy's 


FAKADAY.  409 

opposition  to  the   election   of  Faraday  to  the  Royal 
Society  is,  I  am  persuaded,  to  be  ascribed. 

These  matters  are  touched  upon  with  perfect  candour, 
and  becomiug  consideration,  in  the  volumes  of  Dr. 
Bence  Jones ;  but  in  '  society '  they  are  not  always  so 
handled.  Here  a  name  of  noble  intellectual  associations 
is  surrounded  by  injurious  rumours  which  I  would  will- 
ingly scatter  for  ever.  The  pupil's  magnitude,  and 
the  splendour  of  his  position,  are  too  great  and  absolute 
to  need  as  a  foil  the  humiliation  of  his  master.  Brothers 
in  intellect,  Davy  and  Faraday,  however,  could  never 
have  become  brothers  in  feeling;  their  characters  were 
too  unlike.  Davy  loved  the  pomp  and  circumstance  of 
fame ;  Faraday  the  inrer  consciousness  that  he  had 
fairly  won  renown.  They  were  both  proud  men.  But 
with  Davy  pride  projected  itself  into  the  outer  world ; 
while  with  Faraday  it  became  a  steadying  and  dignifying 
inward  force.  In  one  great  particular  they  agreed. 
Each  of  them  could  have  turned  his  science  to  immense 
commercial  profit,  but  neither  of  them  did  so.  The 
noble  excitement  of  research,  and  the  delight  of  dis- 
covery, constituted  their  reward.  I  commend  them  to 
the  reverence  which  great  gifts  greatly  exercised  ought 
to  inspire.  They  were  both  ours;  and  through  the 
coming  centuries  England  will  be  able  to  point  with 
just  pride  to  the  possession  of  such  men. 


The  first  volume  of  the  *Life  and  Letters'  reveals 
to  us  the  youth  who  was  to  be  father  to  the  man. 
Skilful,  aspiring,  resolute,  he  grew  steadily  in  know- 
ledge and  in  power.  Consciously  or  unconsciously,  the 
relation   of  Action  to   Reaction  was   ever  present  to 


410  FRAGMENTS  OF  SCIENCE. 

Faraday's  mind.  It  had  been  fostered  by  his  discovery 
of  Mao^netic  Kotations,  and  it  planted  in  him  more 
daring  ideas  of  a  similar  kind.  Magnetism  he  knew 
could  be  evoked  by  electricity,  and  he  thought  that 
electricity,  in  its  turn,  ought  to  be  capable  of  evolution 
by  magnetism.  On  August  29,  1831,  his  experiments 
on  this  subject  began.  He  had  been  fortified  by 
previous  trials,  which,  though  failures,  had  begotten 
instincts  directing  him  towards  the  truth.  He,  like 
every  strong  worker,  might  at  times  miss  the  outward 
object,  but  he  always  gained  the  inner  light,  education, 
and  expansion.  Of  this  Faraday's  life  was  a  constant 
illustration.  By  November  he  had  discovered  and  col- 
ligated a  multitude  of  the  most  wonderful  and  un- 
expected phenomena.  He  had  generated  currents  by 
currents ;  currents  by  magnets,  permanent  and  transi- 
tory; and  he  afterwards  generated  currents  by  the 
earth  itself.  Arago's  '  Magnetism  of  Eotation,'  which 
had  for  years  offered  itself  as  a  challenge  to  the  best 
scientific  intellects  of  Europe,  now  fell  into  his  hands. 
It  proved  to  be  a  beautiful,  but  still  special,  illustration 
of  the  great  principle  of  Magneto-electric  Induction. 
Nothing  equal  to  this  latter,  in  the  way  of  pure  experi- 
mental enquiry,  had  previously  been  achieved. 

Electricities  from  various  sources  were  next  exa- 
mined, and  their  differences  and  resemblances  revealed. 
He  thus  assured  himself  of  their  substantial  identity. 
He  then  took  up  Conduction,  and  gave  many  striking 
illustrations  of  the  influence  of  Fusion  on  Conducting 
Power.  Renouncing  professional  work,  from  which  at 
ihis  time  he  might  have  derived  an  income  of  many 
thousands  a  year,  he  poured  his  whole  momentum  into 
his  researches.  He  was  long  entangled  in  Electro- 
chemistry. The  light  of  law  was  for  a  time  obscured 
by  the  thick  umbrage  of  novel  facts;  but  he  finally 


FARADAY.  41 1 

emerged  from  his  researches  with  the  great  principle  of 
Definite  Electro-chemical  Decomposition  in  his  hands. 
If  his  discovery  of  Magneto-electricity  may  be  ranked 
with  that  of  the  pile  by  Volta,  this  new  discovery 
may  almost  stand  beside  that  of  Definite  Combining 
Proportions  in  Chemistry.  He  passed  on  to  Static 
Electricity — its  Conduction,  Induction,  and  Mode  of 
Propagation.  He  discovered  and  illustrated  the  prin- 
ciple of  Inductive  Capacity  ;  and,  turning  to  theory,  he 
asked  himself  how  electrical  attractions  and  repulsions 
are  transmitted.  Are  they,  like  gravity,  actions  at  a 
distance,  or  do  they  require  a  medium  ?  If  the  former, 
then,  like  gravity,  they  will  act  in  straight  lines ;  if 
the  latter,  then,  like  sound  or  light,  they  may  turn  a 
corner.  Faraday  held — and  his  views  are  gaining 
ground — that  his  experiments  proved  the  fact  of  curvi- 
linear propagation,  and  hence  the  operation  of  a  medium. 
Others  denied  this  ;  but  none  can  deny  the  profound 
and  philosophic  character  of  his  leading  thought.^  The 
first  volume  of  the  Kesearches  contains  all  the  papers 
here  referred  to. 

Faraday  had  heard  it  stated  that  henceforth  physical 
discoveries  would  be  made  solely  by  the  aid  of  mathe- 
matics ;  that  we  had  our  data,  and  needed  only  to  work 
deductively.  Statements  of  a  similar  character  crop 
out  from  time  to  time  in  our  day.  They  arise  from 
an  imperfect  acquaintance  with  the  nature,  present 
condition,  and  prospective  vastness  of  the  field  ot 
physical  enquiry.  The  tendency  of  natural  science 
doubtless  is  to  bring  all  physical  phenomena  under  the 
dominion  of  mechanical  laws ;  to  give  them,  in  other 
words,  mathematical  expression.     But  our  approach  to 

'  In  a  very  remarkable  paper  published  in  PoggendorfE's 
« Annalen '  for  1857,  Werner  Siemens  accepts  and  develops  Faraday's 
theory  of  Molecular  Induction. 


412  FRAaMENTS  OF  SCIENCE. 

this  result  is  asymptotic  ;  and  for  ages  to  come — pos- 
sibly for  all  the  ages  of  the  human  race — Nature  will 
find  room  for  both  the  philosophical  experim^^iiter  and 
the  mathematician.  Faraday  entered  his  protest  against 
the  foregoing  statement  by  labelling  his  investigations 
*  Experimental  Eesearches  in  Electricity.'  They  were 
completed  in  1854,  and  three  volumes  of  them  have 
been  published.  For  the  sake  of  reference,  he  numbered 
every  paragraph,  the  last  number  being  3362,  In 
1859  he  collected  and  published  a  fourth  volume 
of  papers,  under  the  title,  'Experimental  Researches 
in  Chemistry  and  Physics.*  Thus  did  this  apostle  of 
experiment  illustrate  its  power,  and  magnify  his 
office. 

The  second  volume  of  the  Researches  embraces 
memoirs  on  the  Electricity  of  the  Grymnotus  ;  on  the 
Source  of  Power  in  the  Voltaic  Pile ;  on  the  Electricity 
evolved  by  the  Friction  of  Water  and  Steam,  in  which 
the  phenomena  and  principles  of  Sir  William  Arm- 
strong's Hydro-electric  machine  are  described  and  de- 
veloped ;  a  paper  on  Magnetic  Rotations,  and  Faraday's 
letters  in  relation  to  the  controversy  it  aroused.  The 
contribution  of  most  permanent  value  here,  is  that  on 
the  Source  of  Power  in  the  Voltaic  Pile.  By  it  the 
Contact  Theory,  pure  and  simple,  was  totally  over- 
thrown, and  the  necessity  of  chemical  action  to  the 
maintenance  of  the  current  demonstrated. 

The  third  volume  of  the  Researches  opens  with  a 
memoir  entitled  '  The  Magnetisation  of  Light,'  and  the 
'  Illumination  of  Magnetic  Lines  of  Force.'  It  is  diffi- 
cult even  now  to  affix  a  definite  meaning  to  this  title  ; 
tut  the  discovery  of  the  rotation  of  the  plane  of  polari- 
sation, which  it  announced,  seems  pregnant  with  great 
results.  The  writings  of  William  Thomson  on  the 
theoretic  aspects  of  the  discovery ;  the  excellent  electro- 


FARADAY.  413 

dynamic  measurements  of  Wilhelm  Weber,  which  are 
models  of  experimental  completeness  and  skill ;  Weber's 
labours  in  conjunction  with  his  lamented  friend  Kohl- 
rausch — above  all,  the  researches  of  Clerk  Maxwell  on 
the  Electro-magnetic  Theory  of  Light — point  to  that 
wonderful  and  mysterious  medium,  which  is  the  vehicle 
of  light  and  radiant  heat,  as  the  probable  basis  also  of 
magnetic  and  electric  phenomena.  The  hope  of  such  a 
connection  was  first  raised  by  the  discovery  here  referred 
to.*  Faraday  himself  seemed  to  cling  with  particular 
affection  to  this  discovery.  He  felt  that  there  was 
more  in  it  than  he  was  able  to  unfold.  He  predicted 
that  it  would  grow  in  meaning  with  the  growth  of 
science.  This  it  has  done  ;  this  it  is  doing  now.  Its 
right  interpretation  will  probably  mark  an  epoch  in 
scientific  history. 

Eapidly  following  it  is  the  discovery  of  Diamag- 
netism,  or  the  repulsion  of  matter  by  a  magnet.  Brug- 
mans  had  shown  that  bismuth  repelled  a  magnetic 
needle.  Here  he  stopped.  Le  Bailliff  proved  that 
antimony  did  the  same.  Here  he  stopped.  Seebeck, 
Becquerel,  and  others,  also  touched  the  discovery. 
Tliese  fragmentary  gleams  excited  a  momentaiy  curiosity 
and  were  almost  forgotten,  when  Faraday  independently 
alighted  on  the  same  facts ;  and,  instead  of  stopping, 
made  them  the  inlets  to   a   new   and  vast  region  of 

*  A  letter  addressed  to  me  by  Professor  Weber  on  March 
18  last  contains  the  following  reference  to  the  connection  here 
mentioned :  *  Die  Hoffnung  einer  solchen  Combination  ist  durch 
Faraday's  Entdeckung  der  Drehung  der  Polarisationsebene  durch 
magnetische  Direct ionskraft  zuerst,und  sodann  durch  die  Ueberein- 
stimmung  derjenigen  Geschwindigkeit,  welche  das  Verhiiltniss  der 
electro-dynamischen  Einbeit  zur  electro-statischen  ausdriickt,  mft 
der  Geschwindigkeit  des  Lichts  angeregt  worden  ;  und  mir  scheint 
von  alien  Versuchen,  welche  zur  Verwirklicbung  dieser  Hoffnung 
gemacht  worden  sind,  das  von  Herrn  Maxwell  gemachte  am 
erfolgreichsten.* 


414  FKAGMENTS    OF    SCIENCE. 

research.  The  value  of  a  discovery  is  to  be  measured 
by  the  intellectual  action  it  calls  forth ;  and  it  wag 
Faraday's  good  fortune  to  strike  such  lodes  of  scientific 
truth  as  give  occupation  to  some  of  the  best  intellects 
of  our  age. 

The  salient  quality  of  Faraday's  scientific  character 
reveals  itself  from  beginning  to  end  of  theae  volumes ; 
a  union  of  ardour  and  patience — the  one  prompting 
the  attack,  the  other  holding  him  on  to  it,  till  defeat 
was  final  or  victory  assured.  Certainty  in  one  sense  or 
the  other  was  necessary  to  his  peace  of  mind.  The 
right  method  of  investigation  is  perhaps  incommuni- 
cable ;  it  depends  on  the  individual  rather  than  on  the 
system,  and  the  mark  is  missed  when  Faraday's  re- 
searches are  pointed  to  as  merely  illustrative  of  the 
power  of  the  inductive  philosophy.  The  brain  may  be 
filled  with  that  philosophy ;  but  without  the  energy 
and  insight  which  this  man  possessed,  and  which  with 
him  were  personal  and  distinctive,  we  should  never  rise 
to  the  level  of  his  achievements.  His  power  is  that  of 
individual  genius,  rather  than  of  philosophic  method ; 
the  energy  of  a  strong  soul  expressing  itself  after  its 
own  fashion,  and  acknowledging  no  mediator  between 
it  and  Nature. 

The  second  volume  of  the  '  Life  and  Letters,'  like 
the  first,  is  a  historic  treasury  as  regards  Faraday's 
work  and  character,  and  his  scientific  and  social  rela- 
tions. It  contains  letters  from  Humboldt,  Herschel, 
Hachette,  De  la  Rive,  Dumas,  Liebig,  Melloni,  Bec- 
querel.  Oersted,  Pliicker,  Du  Bois  Eeymond,  Lord 
Melbourne,  Prince  Loius  Napoleon,  and  many  other 
distinguished  men.  I  notice  with  particular  pleasure  a 
Iqjbter  from  Sir  John  Herschel,  in  reply  to  a  sealed 
packet  addressed  to  him  by  Faraday,  but  which  he  had 
permission  to  open  if  he  pleased.     The  packet  referred 


FAKADAY.  415 

U)  one  of  the  many  unfulfilled  hopes  which  spring  up 
in  the  minds  of  fertile  investigators  : — 

'  Gro  on  and  prosper,  "  from  strength  to  strengthj" 
like  a  victor  marching  with  assured  step  to  further 
conquests ;  and  be  certain  that  no  voice  will  join  more 
heartily  in  the  peans  that  already  begin  to  rise,  and 
will  speedily  swell  into  a  shout  of  triumph,  astounding 
even  to  yourself,  than  that  of  J.  F.  W.  Herschel.' 

Faraday's  behaviour  to  Melloni  in  1835  merits  a 
word  of  notice.  The  young  man  was  a  political  exile 
in  Paris.  He  had  newly  fashioned  and  applied  the 
thermo-electric  pile,  and  had  obtained  with  it  results 
of  the  greatest  importance.  But  they  were  not  appre- 
ciated. With  the  sickness  of  disappointed  hope  Melloni 
waited  for  the  report  of  the  Commissioners,  appointed 
by  the  Academy  of  Sciences  to  examine  the  Primier. 
At  length  he  published  his  researches  in  the  '  Aunales 
de  Chimie.'  They  thus  fell  into  the  hands  of  Faraday, 
who,  discerning  at  once  their  extraordinary  merit, 
obtained  for  their  author  the  Eumford  Medal  of  the 
Eoyal  Society.  A  sum  of  money  always  accompanies 
this  medal ;  and  the  pecuniary  help  was,  at  this  time, 
even  more  essential  than  the  mark  of  honour  to  the 
young  refugee.     Melloni's  gratitude  was  boundless  : — 

'Et  vous,  monsieur,'  he  writes  to  Faraday,  'qui 
appartenez  a  une  societe  a  laquelle  je  n'avais  rien  oflfert, 
vous  qui  me  connaissiez  a  peine  de  nom  ;  vous  n'avez 
pas  demande  si  j'avaisdes  ennemis  faibles  ou  puissants, 
ni  calcule  quel  en  etait  le  nombre ;  mais  vous  avez 
parle  pour  I'opprime  etranger,  pour  celui  qui  n'avait 
pas  le  moindre  droit  a  tant  de  bienveillance,  et  vos 
paroles  ont  ete  accueillies  favorablement  par  des  col- 
logues consciencieux !  Je  reconnais  bien  la  des  homraea 
dignes  de  leur  noble  mission,  les  veritable  represen- 
tants  de  la  science  d'un  pays  libre  et  genereux.' 


416  FRAGMENTS  OF  SCIENCE. 

Within  the  prescribed  limits  of  this  article  it  would 
be  impossible  to  give  even  the  slenderest  summary  of 
Faraday's  correspondence,  or  to  carve  from  it  more  than 
the  merest  fragments  of  his  character.  His  letters, 
written  to  Lord  Melbourne  and  others  in  1836,  regard- 
ing his  pension,  illustrate  his  uncompromising  independ- 
ence. The  Prime  Minister  had  offended  him,  but 
assuredly  the  apology  demanded  and  given  was  com- 
plete. I  think  it  certain  that,  notwithstanding  the 
very  full  account  of  this  transaction  given  by  Dr.  Bence 
Jones,  motives  and  influences  were  at  work  which  even 
now  are  not  entirely  revealed.  The  minister  was  bit- 
terly attacked,  but  he  bore  the  censure  of  the  press  with 
great  dignity.  Faraday,  while  he  disavowed  having 
either  directly  or  indirectly  furnished  the  matter  of 
those  attacks,  did  not  publicly  exonerate  the  Premier. 
The  Hon.  Caroline  Fox  had  proved  herself  Faraday's 
ardent  friend,  and  it  was  she  who  had  healed  the  breach 
between  the  philosopher  and  the  minister.  She  mani 
festly  thought  that  Faraday  ought  to  have  come  for- 
ward in  Lord  Melbourne's  defence,  and  there  is  a  flavour 
of  resentment  in  one  of  her  letters  to  him  on  the  sub- 
ject. No  doubt  Faraday  had  good  grounds  for  his 
reticence,  but  they  are  to  me  unknown. 

In  1841  his  health  broke  down  utterly,  and  he  went 
to  Switzerland  with  his  wife  and  brother-in-law.  His 
bodily  vigour  soon  revived,  and  he  accomplished  feats 
of  walking  respectable  even  for  a  trained  mountaineer. 
The  published  extracts  from  his  Swiss  journal  contain 
many  beautiful  and  touching  allusions.  Amid  references 
to  the  tints  of  the  Jungfrau,  the  blue  rifts  of  the  glaciers, 
and  the  noble  Niesen  towering  over  the  Lake  of  Thun, 
we  come  upon  the  charming  little  scrap  wliich  I  have 
elsewhere  quoted :  '  Clout-nail  making  goes  on  here 
rather   considerably,  and   is   a   very   neat  and   pretty 


FARADAY.  41 V 

Operation  to  observe.  I  love  a  smith's  shop  and  any- 
thing relating  to  smithery.  My  father  was  a  smith.' 
This  is  from  his  journal ;  but  he  is  unconsciously  speak- 
ing to  somebody — perhaps  to  the  world. 

His  description  of  the  Staubbach,  Griessbach,  and 
of  the  scenic  effects  of  sky.  and  mountain,  are  all  fine 
and  sympathetic.  But  amid  it  all,  and  in  reference  to 
it  all,  he  tells  his  sister  that  '  true  enjoyment  is  from 
within,  not  from  without.'  In  those  days  Agassiz  was 
living  under  a  slab  of  gneiss  on  the  glacier  of  the  Aar. 
Faraday  met  Forbes  at  the  Grrimsel,  and  arranged 
with  him  an  excursion  to  the  '  Hotel  des  Neuchatelois ' ; 
but  indisposition  put  the  project  out. 

From  the  Fort  of  Ham,  in  1843,  Faraday  received 
a  letter  addressed  to  him  by  Prince  Louis  Napoleon 
Bonaparte.  He  read  this  letter  to  me  many  years  ago, 
and  the  desire,  shown  in  various  ways  by  the  French 
Emperor,  to  turn  modern  science  to  account,  has  often 
reminded  me  of  it  since.  At  the  age  of  thirty-five  the 
prisoner  of  Ham  speaks  of  'rendering  his  captivity 
less  sad  by  studying  the  great  discoveries'  which 
science  owes  to  Faraday ;  and  he  asks  a  question  which 
reveals  his  cast  of  thought  at  the  time  :  '  What  is  the 
most  simple  combination  to  give  to  a  voltaic  battery, 
in  order  to  produce  a  spark  capable  of  setting  fire  to 
powder  under  water  or  under  ground?'  Should  the 
necessity  arise,  the  French  Emperor  will  not  lack  at 
the  outset  the  best  appliances  of  modern  science  ;  while 
we,  I  fear,  shall  have  to  learn  the  magnitude  of  the 
resources  we  are  now  neglecting  amid  the  pangs  of 
actual  war.* 

*  The  'science  '  has  since  been  applied,  with  astonishing  effect, 
by  those  who  liad  sludied  it  far  more  thoroughly  than  the  Emperor 
of  the  French.  We  also,  1  am  happy  to  think,  have  improved  the 
time  since  the  above  woid:s  were  written  [1878]. 


418  FRAGMENTS  OF  SCIENCE, 

One  turns  with  renewed  pleasure  to  Faraday's  letters 
to  his  wife,  published  in  the  second  volume.  Here 
surely  the  loving  essence  of  the  man  appears  more  dis- 
tinctly than  anywhere  else.  From  the  house  of  Dr. 
Percy,  in  Birmingham,  he  writes  thus  : — 

*  Here — even  here — the  moment  I  leave  the  table, 
I  wish  I  were  with  you  in  quiet.     Oh,  what  happiness 
is  ours  I     My  runs  into  the  world  in  this  way  only  serve 
to  make  me  esteem  that  happiness  the  more.' 
And  again : 

'We  have  been  to  a  grand  conversazione  in  the 
town-hall,  and  I  have  now  returned  to  my  room  to 
talk  with  you,  as  the  pleasantest  and  happiest  thing 
that  I  can  do.  Nothing  rests  me  so  much  as  com- 
munion with  you.  I  feel  it  even  now  as  I  write,  and 
catch  myself  saying  the  words  aloud  as  I  write  them.' 
Take  this,  moreover,  as  indicative  of  his  love  for 
Nature : 

'  After  writing,  I  walk  out  in  the  evening  hand  in 
hand  with  my  dear  wife  to  enjoy  the  sunset ;  for  to 
me  who  love  scenery,  of  all  that  I  have  seen  or  can  see, 
there  is  none  surpasses  that  of  heaven.  A  glorious  sun- 
set brings  with  it  a  thousand  thoughts  that  delight  me.' 

Of  the  numberless  lights  thrown  upon  him  by  the 
'  Life  and  Letters,'  some  fall  upon  his  religion.  In  a 
letter  to  Lady  Lovelace,  he  describes  himself  as  belong- 
ing to  'a  very  small  and  despised  sect  of  Christians, 
known,  if  known  at  all,  as  Sandemanians,  and  our 
hope  is  founded  on  the  faith  that  is  in  Christ.'  He 
adds :  '  I  do  not  think  it  at  all  necessary  to  tie  the  study 
of  the  natural  sciences  and  religion  together,  and  in 
my  intercourse  with  my  fellow-creatures,  that  which 
is  religious,  and  that  which  is  philosophical,  have  ever 
been  two  distinct  things.'  He  saw  clearly  the  danger  of 
quitting  his  moorings,  and  his  science  acted  indirectly 


FARADAY.  419 

as  the  safeguard  of  his  faith.  For  his  investigations 
80  filled  his  mind  as  to  leave  no  room  for.  sceptical 
questionings,  thus  shielding  from  the  assaults  of  philo- 
sophy the  creed  of  his  youth.  His  religion  was  con- 
stitutional and  hereditary.  It  was  implied  in  the 
eddies  of  his  blood  and  in  the  tremors  of  his  brain  ;  and, 
however  its  outward  and  visible  form  might  have 
changed,  Faraday  would  still  have  possessed  its  elemental 
constituents — awe,  reverence,  truth,  and  love. 

It  is  worth  enquiring  how  so  profoundly  religious  a 
mind,  and  so  great  a  teacher,  would  be  likely  to  regard 
our  present  discussions  on  the  subject  of  education. 
Faraday  would  be  a  '  secularist '  were  he  now  alive. 
He  had  no  sj^mpathy  with  those  who  contemn  know- 
ledge unless  it  be  accompanied  by  dogma.  A  lecture 
delivered  before  the  City  Philosophical  Society  in  1818, 
when  he  was  twenty-six  years  of  age,  expresses  the 
views  regarding  education  which  he  entertained  to  the 
end  of  his  life.  '  First,  then,'  he  says,  'all  theological 
considerations  are  banished  from  the  society,  and  of 
course  from  my  remarks  ;  and  whatever  I  may  say  has 
no  reference  to  a  future  state,  or  to  the  means  which 
are  to  be  adopted  in  tliis  world  in  anticipation  of  it. 
Next,  I  have  no  intention  of  substituting  anything  for 
religion,  but  I  wish  to  take  that  part  of  human  nature 
which  is  independent  of  it.  Morality,  philosophy, 
commerce,  the  various  institutions  and  habits  of 
society,  are  independent  of  religion,  and  may  exist 
either  with  or  without  it.  They  are  always  the  same, 
and  can  dwell  alike  in  the  breasts  of  those  who,  from 
opinion,  are  entirely  opposed  in  the  set  of  principles 
they  include  in  the  term  religion,  or  in  those  who  have 
none. 

*  To  discriminate  more  closely,  if  possible,  I  will 
observe   that   we    have    no    right  to   judge  religious 


420  FBAGMENTS  OF  SCIENCK 

opinions ;  but  the  human  nature  of  this  evening;  is 
that  part  of  man  which  we  have  a  right  to  judge. 
And  I  think  it  will  be  found  on  examination,  that  this 
humanity — as  it  may  perhaps  be  called — will  accord 
with  what  I  have  before  described  as  being  in  our  own 
hands  so  improvable  and  perfectible.' 

In  an  old  journal  I  find  the  following  remarks  on 
one  of  my  earliest  dinners  with  Faraday:  'At  tsvo 
o'clock  he  came  down  for  me.  He,  his  niece,  and 
myself,  formed  the  party,  "  I  never  give  dinners,"  he 
said.  "  I  don't  know  how  to  give  dinners,  and  I 
never  dine  out.  But  I  should  not  like  my  friends  to 
attribute  this  to  a  wrong  cause.  I  act  thus  for  the 
sake  of  securing  time  for  work,  and  not  through  re- 
ligious motives,  as  some  imagine."  He  said  grace.  I 
am  almost  ashamed  to  call  his  pi'ayer  a  ^'  saying  of 
grace."  In  the  language  of  Scripture,  it  might  be 
described  as  the  petition  of  a  son,  into  whose  heart 
God  had  sent  the  Spirit  of  His  Son,  and  who  with 
absolute  trust  asked  a  blessing  from  his  father.  We 
dined  on  roast  beef,  Yorkshire  pudding,  and  potatoes ; 
drank  sherry,  talked  of  research  and  its  requirements, 
and  of  his  liabit  of  keeping  himself  free  from  the  dis- 
tractions of  society.  He  was  bright  and  joyful — boy- 
like, in  fact,  though  he  is  now  sixty- two.  His  work 
excites  admiration,  but  contact  with  him  warms  and 
elevates  the  heart.  Here,  surely,  is  a  strong  man.  I 
love  strength ;  but  let  me  not  forget  the  example  of 
its  union  with  modesty,  tenderness,  and  sweetness,  in 
the  character  of  Faraday.' 

Faraday's  progress  in  discovery,  and  the  salient 
points  of  his  character,  are  well  brought  out  by  the 
wise  choice  of  letters  and  extracts  published  in  the 
volumes  before  us.  I  will  not  call  the  labours  of  the 
biographer  final.     So  great  a  character  will  challenge 


FAKADAY.  421 

reconstruction.  In  the  coming  time  some  sympathetic 
spirit,  with  the  requisite  strength,  knowledge,  and 
solvent  power,  will,  I  doubt  not,  render  these  materials 
plastic,  give  them  more  perfect  organic  form,  and  send 
through  them,  with  less  of  interruption,  the  currents  of 
Faraday's  life.  '  He  was  too  good  a  man,'  writes  his 
present  biographer,  '  for  me  to  estimate  rightly,  and  too 
great  a  philosopher  for  me  to  understand  thoroughly.' 
That  may  be :  but  the  reverent  affection  to  which  we 
owe  the  discovery,  selection,  and  arrangement  of  the 
materials  here  placed  before  us,  is  probably  a  surer 
guide  than  mere  literary  skill.  The  task  of  the  artist 
who  may  wish  in  future  times  to  reproduce  the  real 
though  unobtrusive  grandeur,  the  purity,  beauty,  and 
childlike  simplicity  of  him  whom  we  have  lost,  will 
find  his  chief  treasury  already  provided  for  him  by 
Dr.  Bence  Jones's  labour  of  love. 


422  FKAGMENTS  OF  SCIENCE. 


XIX. 

THE   COPLEY  MEDALIST  OF  1870. 

rilHIRTY  years  ago  Electro-magnetism  was  looked  to 
X  as  a  motive  power,  which  might  possibly  com- 
pete with  steam.  In  centres  of  industry,  such  as 
Manchester,  attempts  to  investigate  and  apply  this 
power  were  numerous.  This  is  shown  by  the  scientific 
literature  of  the  time.  Among  others  Mr.  James 
Prescot  Joule,  a  resident  of  Manchester,  took  up  the 
subject,  and,  in  a  series  of  papers  published  in  Stur- 
geon's *  Annals  of  Electricity'  between  1839  and  1841, 
described  various  attempts  at  the  construction  and  per- 
fection of  electro-magnetic  engines.  Tlie  spirit  in 
which  Mr.  Joule  pursued  these  enquiries  is  revealed  in 
the  following  extract :  '  I  am  particularly  anxious,'  he 
says,  '  to  communicate  any  new  arrangement  in  order, 
if  possible,  to  forestall  the  monopolising  designs  of  those 
who  seem  to  regard  this  most  interesting  subject  merely 
in  the  light  of  pecuniary  speculation.'  He  was  natur- 
ally led  to  investigate  the  laws  of  electro-magnetic 
attractions,  and  in  1840  he  announced  the  important 
principle  that  the  attractive  force  exerted  by  two  electro- 
magnets, or  by  an  electro-magnet  and  a  mass  of  an- 
nealed iron,  is  directly  proportional  to  the  square 
of  the  strength  of  the  magnetising  current;  while 
the  attraction  exerted  between  an  electro-magnet 
and  the   pole   of  a   permanent   steel    magnet,   varies 


THE  COPLEY  MEDALIST  OF   1870.  423 

simply  as  the  strength  of  the  current.  These  inves- 
tigations were  conducted  independently  of,  though  a 
little  subsequently  to,  the  celebrated  enquiries  of 
Henry,  Jacobi,  and  Lenz  and  Jacobi,  on  the  same 
subject. 

On  December  17,  1840,  Mr.  Joule  communicated  to 
the  Eoyal  Society  a  paper  on  the  production  of  heat 
by  Voltaic  electricity.  In  it  he  announced  the  law  that 
the  calorific  effects  of  equal  quantities  of  transmitted 
electricity  are  proportional  to  the  resistance  overcome 
by  the  current,  whatever  may  be  the  length,  thickness, 
shape,  or  character  of  the  metal  which  closes  the  circuit ; 
and  also  proportional  to  the  square  of  the  quantity  of 
transmitted  electricity.  This  is  a  law  of  primary 
importance.  In  another  paper,  presented  to,  but 
declined  by,  the  Eoyal  Society,  he  confirmed  this  law 
by  new  experiments,  and  materially  extended  it.  He 
also  executed  experiments  on  the  heat  consequent  on 
the  passage  of  Voltaic  electricity  through  electro- 
lytes, and  found,  in  all  cases,  that  the  heat  evolved 
by  the  proper  action  of  any  Voltaic  current  is  propor- 
tional to  the  square  of  the  intensity  of  that  current, 
multiplied  by  the  resistance  to  conduction  which 
it  experiences.  From  this  law  he  deduced  a  number 
of  conclusions  of  the  highest  importance  to  electro- 
chemistry. 

It  was  during  these  enquiries,  which  are  marked 
throughout  by  rare  sagacity  and  originality,  that  the 
great  idea  of  establishing  quantitative  relations  between 
Mechanical  Energy  and  Heat  arose  and  assumed  definite 
form  in  his  mind.  In  1843  Mr.  Joule  read  before  the 
meeting  of  the  British  Association  at  Cork  a  paper '  On 
the  Calorific  Effects  of  Magneto-Electricity,  and  on  the 
Mechanical  Value  of  Heat.'  Even  at  the  present  day 
this  memoir  is  tough  reading,  and  at  the  time  it  was 

28 


424  FKAGMEJSTS  OF  SCIENCE. 

written  it  must  have  appeared  hopelessly  entangled. 
This,  I  should  think,  was  the  reason  why  P'araday 
advised  Mr.  Joule  not  to  submit  the  paper  to  the 
Eoyal  Society.  But  its  drift  and  results  are  sumiaed 
up  in  these  memorable  words  by  its  author,  written 
some  time  subsequently ;  '  In  that  paper  it  was  demon- 
strated experimentally,  that  the  mechanical  power 
exerted  in  turning  a  magneto-electric  macliine  is  con- 
verted into  the  heat  evolved  by  the  passage  of  the 
currents  of  induction  through  its  coils;  and,  on  the 
other  hand,  that  the  motive  power  of  the  electro- 
magnetic engine  is  obtained  at  the  expense  of  the  heat 
due  to  the  chemical  reaction  of  the  battery  by  which  it 
is  worked.' '  It  is  needless  to  dwell  upon  the  weight 
and  importance  of  this  statement. 

Considering  the  imperfections  incidental  to  a  first 
determination,  it  is  not  surprising  that  the  'mechani- 
cal values  of  heat,'  deduced  from  the  different  series 
of  experiments  published  in  1843,  varied  widely 
from  each  other.  The  lowest  limit  was  587,  and 
the  highest  1,026  foot-pounds,  for  1°  Fahr.  of  tempera- 
ture. 

One  noteworthy  result  of  his  enquiries,  which  was 
pointed  out  at  the  time  by  Mr.  Joule,  hi.d  reference  to 
the  exceedingly  small  fraction  of  the  heat  actually 
converted  into  useful  efifect  in  the  steam-engine.  The 
thoughts  of  the  celebrated  Julius  Eobert  Mayer,  who 
was  then  engaged  in  Germany  upon  the  same  question, 
had  moved  independently  in  the  same  groove  ;  but  to 
his  labours  due  reference  will  be  made  on  a  future 
occasion.^  In  the  memoir  now  referred  to,  Mr.  Joule 
also  announced  that  he  had  proved  heat  to  be  evolved 
during  the  passage  of  water  through  narrow  tubes  ;  and 

•  Phil.  Mag.,  May  1845.  •  See  the  next  Fragment. 


THE  COPLEY  MEDALIST  OF  1870.  425 

he  deduced  from  these  experiments  an  equivalent  of 
770  foot-pounds,  a  figure  remarkably  near  the  one  now 
accepted.  A  detached  statement  regarding  the  origin 
and  convertibility  of  animal  heat  strikingly  illustrates 
the  penetration  of  Mr.  Joule,  and  his  mastery  of  prin- 
ciples, at  the  period  now  referred  to.  A  friend  had 
mentioned  to  him  Haller's  liypothesis,  that  animal  heat 
might  arise  from  the  friction  of  the  blood  in  the  veins 
and  arteries.  '  It  is  unquestionable,'  writes  Mr.  Joule, 
'  that  heat  is  produced  by  such  friction  ;  but  it  must  be 
understood  that  the  mechanical  force  expended  in  the 
friction  is  a  part  of  the  force  of  affinity  which  causes 
the  venous  blood  to  unite  with  oxygen,  so  that  the 
whole  heat  of  the  system  must  still  be  referred  to  the 
chemical  changes.  But  if  the  animal  were  engaged  in 
turning  a  piece  of  machinery,  or  in  ascending  a  moun- 
tain, I  apprehend  that  in  proportion  to  the  muscular 
effort  put  forth  for  the  purpose,  a  diminution  of  the 
heat  evolved  in  the  system  by  a  given  chemical  action 
would  be  experienced.'  The  italics  in  this  memorable 
passage,  written,  it  is  to  be  remembered,  in  1843,  are 
Mr.  Joule's  own. 

The  concluding  paragraph  of  this  British  Association 
paper  equally  illustrates  his  insight  and  precision, 
regarding  the  nature  of  chemical  and  latent  heat.  '  I 
had,'  he  writes,  '  endeavoin-ed  to  prove  that  when  two 
atoms  combine  together,  the  heat  evolved  is  exactly 
that  which  would  have  been  evolved  by  the  electrical 
current  due  to  the  chemical  action  taking  place,  and  is 
therefore  proportional  to  the  intensity  of  the  chemical 
force  causing  the  atoms  to  combine.  I  now  venture  to 
state  more  explicitly,  that  it  is  not  precisely  the  attrac- 
tion of  affinity,  but  rather  the  mechanical  force  ex- 
pended by  the  atoms  in  falling  towards  one  another. 


426  FRAGMENTS  OF  SCIENCE. 

which  determines  the  intensity  of  the  current,  and, 
consequently,  the  quantity  of  heat  evolved ;  so  that  we 
have  a  simple  hypothesis  by  which  we  may  explain  why 
heat  is  evolved  so  freely  in  the  combination  of  gases, 
and  by  which  indeed  we  may  account  "  latent  heat  "  as 
a  mechanical  power,  prepared  for  action,  as  a  watch- 
spring  is  when  5vound  up.  Suppose,  for  the  sake  of 
illustration,  that  8  lbs.  of  oxygen  and  1  lb.  of  hydrogen 
were  presented  to  one  another  in  the  gaseous  state,  and 
then  exploded ;  the  heat  evolved  would  be  about  V 
Fahr.  in  60,000  lbs.  of  water,  indicating  a  mechanical 
force,  expended  in  the  combination,  equal  to  a  weight 
of  about  50,000,000  lbs.  raised  to  the  height  of  one  foot. 
Now  if  the  oxygen  and  hydrogen  could  be  presented 
to  each  other  in  a  liquid  state,  the  heat  of  combina- 
tion would  be  less  than  before,  because  the  atoms 
in  combining  would  fall  through  less  space.'  No 
words  of  mine  are  needed  to  point  out  the  com- 
manding grasp  of  molecular  physics,  in  their  relation 
to  the  mechanical  theory  of  heat,  implied  by  this  state- 
ment. 

Perfectly  assured  of  the  importance  of  the  principle 
which  his  experiments  aimed  at  establishing,  Mr.  Joule 
did  not  rest  content  with  results  presenting  such  discre- 
pancies as  those  above  referred  to.  He  resorted  in  1 844 
to  entirely  new  methods,  and  made  elaborate  experi- 
ments on  the  thermal  changes  produced  in  air  during 
its  expansion  :  firstly,  against  a  pressure,  and  therefore 
performing  work ;  secondly,  against  no  pressure,  and 
therefore  performing  no  work.  He  thus  established 
anew  the  relation  between  the  heat  consumed  and  the 
work  done.  From  five  different  series  of  experiments 
he  deduced  five  different  mechanical  equivalents  ;  the 
agreement  between  them  being  far  greater  than  that 


THE  COPLEY  MEDALIST  OF   1870.  427 

attained  in  his  first  experiments.  The  mean  of  them 
was  802  foot-pounds.  From  experiments  with  water 
agitated  by  a  paddle-wheel,  he  deduced,  in  1845,  an 
equivalent  of  890  foot-pounds.  In  1847  he  again 
operated  upon  water  and  sperm-oil,  agitated  them  by  a 
paddle-wheel,  determined  their  elevation  of  temperature, 
and  the  mechanical  power  which  produced  it.  From 
the  one  he  derived  an  equivalent  of  781*5  foot-pounds; 
from  the  other  an  equivalent  of  782' 1  foot-pounds. 
The  mean  of  these  two  very  close  determinations  is 
781*8  foot-pounds. 

By  this  time  the  labours  of  the  previous  ten  years 
had  made  Mr.  Joule  completely  master  of  the  conditions 
essential  to  accuracy  and  success.  Bringing  his  ripened 
experience  to  bear  upon  the  subject,  he  executed  in 
1849  a  series  of  40  experiments  on  the  friction  of  water, 
50  experiments  on  the  friction  of  mercury,  and  20 
experiments  on  the  friction  of  plates  of  cast-iron.  He 
deduced  from  these  experiments  our  present  mechanical 
equivalent  of  heat,  justly  recognised  all  over  the  world 
as  '  Joule's  equivalent.' 

There  are  labours  so  great  and  so  pregnant  in  conse- 
quences, that  they  are  most  highly  praised  when  they 
are  most  simply  stated.  Such  are  the  labours  of  Mr. 
Joule.  They  constitute  the  experimental  foundation  of 
a  principle  of  incalculable  moment,  not  only  to  the 
practice,  but  still  more  to  the  philosophy  of  Science. 
Since  the  days  of  Newton,  nothing  more  important  than 
the  theory,  of  which  Mr.  Joule  is  the  experimental 
demonstrator,  has  been  enunciated. 

1  have  omitted  all  reference  to  the  numerous  minor 
papers  with  which  Mr.  Joule  has  enriched  scientific 
literature.  Nor  have  I  alluded  to  the  important  inves- 
tigations  which  he   has   conducted  jointly   with   Sir 


428  FRAGMENTS  OF  SCIENCE. 

William  Thomson.  But  sufficient,  I  think,  has  been 
here  said  to  show  that,  in  conferring  upon  Mr.  Joule 
the  highest  honour  of  the  Eoyal  Society,  the  Council 
paid  to  genius  not  only  a  well-won  tribute,  but  one 
which  had  been  fairly  earned  twenty  years  previously.^ 

*  Lord  Beaconsfield  has  recently  honoured  himself  and  England 
by  bestowing  an  annual  pension  of  200/.  on  Dr.  Joule. 


XX. 

THE  COPLEY  MEDALIST  OF  187L 

DR.  JULIUS  EGBERT  MAYER  was  educated  for 
the  medical  profession.  In  the  summer  of  1840, 
as  he  himself  informs  us,  he  was  at  Java,  and  there 
observed  that  the  venous  blood  of  some  of  his  patients 
had  a  singularly  bright  red  colour.  The  observation 
riveted  his  attention ;  he  reasoned  upon  it,  and  came  to 
the  conclusion  that  the  brightness  of  the  colour  was 
due  to  the  fact  that  a  less  amount  of  oxidation  sufficed 
to  keep  up  the  temperature  of  the  body  in  a  hot  climate 
than  in  a  cold  one.  The  darkness  of  the  venous  blood 
he  regarded  as  the  visible  sign  of  the  energy  of  the 
oxidation. 

It  would  be  trivial  to  remark  that  accidents  such  as 
this,  appealing  to  minds  prepared  for  them,  have  often 
led  to  great  discoveries.  Mayer's  attention  was  thereby 
drawn  to  the  whole  question  of  animal  heat.  Lavoisier 
had  ascribed  this  heat  to  the  oxidation  of  the  food. 
'  One  great  principle,'  says  Mayer,  '  of  the  physiological 
theory  of  combustion,  is  that  under  all  circumstances 
the  same  amount  of  fuel  yields,  by  its  perfect  combus- 
tion, the  same  amount  of  heat;  that  this  law  holds  good 
even  for  vital  processes  ;  and  that  hence  the  living  body, 
notwithstanding  all  its  enigmas  and  wonders,  is  incom- 
petent to  generate  heat  out  of  nothing.' 

But  beyond  the  power  of  generating  internal  heat, 


430  FRAGMENTS  OF  SCIENCE. 

the  animal  organism  can  also  generate  heat  outside  of 
itself.  A  blacksmith,  for  example,  by  hammering  can 
heat  a  nail,  and  a  savage  by  friction  can  warm  wood  to 
its  point  of  ignition.  Now,  unless  we  give  up  the  physio- 
logical axiom  that  the  living  body  cannot  create  heat 
out  of  nothing,  '  we  are  driven,'  says  Mayer,  'to  the 
conclusion  that  it  is  the  total  heat  generated  within  and 
without  that  is  to  be  regarded  as  the  true  calorific 
effect  of  the  matter  oxidised  in  the  body.' 

From  this,  again,  he  inferred  that  the  heat  generated 
externally  must  stand  in  a  fixed  relation  to  the  work 
expended  in  its  production.  For,  supposing  the  organic 
processes  to  remain  the  same  ;  if  it  were  possible,  by  the 
mere  alteration  of  the  apparatus,  to  generate  different 
amounts  of  heat  by  the  same  amount  of  work,  it  would 
follow  that  the  oxidation  of  the  same  amount  of  material 
would  sometimes  yield  a  less,  sometimes  a  greater, 
quantity  of  heat.  '  Hence,'  says  Mayer,  '  that  a  fixed 
relation  subsists  between  heat  and  work,  is  a  postulate 
of  the  physiological  theory  of  combustion.' 

This  is  the  simple  and  natural  account,  given  subse- 
quently by  Mayer  liimself,  of  the  course  of  tli ought 
started  by  his  observation  in  Java.  But  the  conviction 
once  formed,  that  an  unalterable  relation  subsists  between 
work  and  heat,  it  was  inevitable  that  Mayer  should  seek 
to  express  it  numerically.  It  was  also  inevitable  that  a 
mind  like  his,  having  raised  itself  to  clearness  on  this 
important  point,  should  push  forward  to  consider  the 
relationship  of  natural  forces  generally.  At  the  begin- 
ning of  1842  his  work  had  made  considerable  progress ; 
but  he  had  become  physician  to  the  town  of  Ileilbronn, 
and  the  duties  of  his  profession  limited  the  time  which 
lie  could  devote  to  purely  scientific  enquiry.  He  thought 
it  wise,  therefore,  to  secure  himself  against  accident,  and 
in  the  spring  of  1842  wrote  to  Liebig,  asking  him  to 


THE  COPLEY  MEDALIST  OF  1871.  431 

publish  in  his  '  Annalen '  a  brief  preliminary  nonce  of 
the  work  then  accomplished.  Liebig  did  so,  and  Dr. 
Mayer's  first  paper  is  contained  in  the  May  number  of 
the 'Annalen'  for  1842. 

Mayer  had  reached  his  conclusions  by  reflecting  on 
the  complex  processes  of  the  living  body ;  but  his  first 
step  in  public  was  to  state  definitely  the  physical  prin- 
ciples on  which  his  physiological  deductions  were  to 
rest.  He  begins,  therefore,  with  the  forces  of  inorganic 
nature.  He  finds  in  the  universe  two  systems  of  causes 
which  are  not  mutually  convertible  ; — the  different  kinds 
of  matter  and  the  different  forms  of  force.  The  first 
quality  of  both  he  affirms  to  be  indestructibility,  A 
force  cannot  become  nothing,  nor  can  it  arise  from 
nothing.  Forces  are  convertible  but  not  destructible. 
In  the  terminology  of  his  time,  he  then  gives  clear  ex- 
pression to  the  ideas  of  potential  and  dynamic  energy, 
illustrating  his  point  by  a  weight  resting  upon  the 
earth,  suspended  at  a  height  above  the  earth,  and  actu- 
ally falling  to  the  earth.  He  next  fixes  his  attention 
on  cases  where  motion  is  apparently  destroyed,  without 
producing  other  motion ;  on  the  shock  of  inelastic 
bodies,  for  example.  Under  what  form  does  the  vanished 
motion  maintain  itself  ?  Experiment  alone,  says  Mayer, 
can  help  us  here.  He  warms  water  by  stirring  it ;  he 
refers  to  the  force  expended  in  overcoming  friction. 
IMotion  in  both  cases  disappears ;  but  heat  is  generated, 
and  the  quantity  generated  is  the  equivalent  of  the 
motion  destroyed.  '  Our  locomotives,'  he  observes  with 
extraordinary  sagacity,  '  may  be  compared  to  distilling 
apparatus  :  the  heat  beneath  the  boiler  passes  into  the 
motion  of  the  train,  and  is  again  deposited  as  heat  in 
the  axles  and  wheels.' 

A  numerical  solution  of  the  relation  between  heat 
and  work  was  what  Mayer  aimed  at,  and  towards  the  end 


432  FRiLGMENTS  OF  SdEKCS. 

of  bis  fiist  paper  he  makes  tiie  attempt.  It  was  known 
that  a  definite  amount  of  air,  in  rising  one  de^^ree  in 
temperature,  can  take  up  two  different  amounts  of  heat. 
If  its  ▼vdnme  he  k^  constant,  it  takes  up  one  amount: 
if  its  presnne  he  kept  constant  it  takes  up  a  different 
smoont.  These  two  amounts  are  caQed  the  specifio 
heat  under  constant  Tdume  and  under  constant  pres- 
sure. Thentioof  the  first  to  the  second  is  as  1  :  1*421. 
No  man,  to  my  knowledge,  prior  to  Dr.  ]klayer,  pene- 
tnted  the  significance  of  these  two  numhers.  He  first 
saw  that  the  excess  0-421  was  not,  as  then  uniyersally 
supposed,  heat  actually  lodged  in  the  gas,  but  beat  which 
had  been  ac^ualty  oMunmed  by  the  gas  in  expanding 
i^ainst  pressuie.  Hie  amount  of  work  here  performed 
was  accurately  known,  the  amount  of  heat  consumed  was 
also  accurate  known,  and  from  these  data  Mayer  deter- 
mined the  mechanical  equivalent  of  heaL  Even  in  this 
first  paper  he  is  able  to  direct  attention  to  the  enormous 
discrepancj  between  the  theoretic  power  of  the  fad 
consumed  in  steam-engines,  and  their  useful  effecL 

Though  this  paper  contains  but  the  germ  of  his 
further  lahooFS,  I  think  it  may  be  safely  assumed  that, 
as  r^;aids  the  mechanical  theoiy  ai  heat,  this  obscure 
Heilbronn  physician,  in  the  year  1842,  was  in  advance 
of  all  the  sri^itific  men  <^  the  time. 

Having,  by  the  publicatiosi  of  this  paper,  secured 
liiinMplf  against  what  he  caDs  *■  ETentoalitaten,'  he  de- 
Toted  every  hour  of  his  spare  time  to  his  studies,  and  in 
1845  pnblidied  a  memoir  which  fiair  transcends  his  first 
one  in  weight  and  fulness,  and,  indeed,  marks  an  epoth 
in  the  h^ory  of  science-  The  title  of  Mayei's  fixst 
paperwas,*  Remarks  on  the  Faces  of  Inoiganic  Nature.* 
The  title  of  his  second  great  es^ay  was, '  Chganie  Motion 
in  its  Connection  with  Nutrition.*  In  it  he  expands  and 
die  physical  prind^es  laid  down  in  Ida  fint 


THE  CX>PLEY  MKOATJOT  OF  ItH.  433 


Inef  paper.  He  goes  feSij  tliroqg^  the  calrwhtinn  cf 
the  mechanical  eqiuYalent  of  beak  He  calcnlates  the 
perfennanees  of  rteam-^nginea,  and  finds  tliai  100  On. 
of  coaly  in  a  good  woddng  engine^  prodnee  onlf  tiw 
same  amoont  of  beat  as  95  lbs.  in  an  unwuitii^  one; 
tbe  5  w>i«an«g  lbs.  barii^  been  qm¥eiled  into  woA, 
He  detennines  tbe  usefol  efedt  of  gunpovdei^and  finds 
mne  per  eent.  of  tiie  fiMPoe  of  llie  eonsmned  diarDaal  In- 
Ycsled  on  the  moYing  haXL  He  leeoids  obaeivaliona  on 
tbe  beat  generated  in  water  agitated  bj  the  pn^ing- 
engine  of  a  paper  manu&ctoiy,  and  ralenlates  tiie  espor- 
talent  of  that  beat  in  boiae-powi^.  He  eon^area 
diemieal  eomfaination  with  nifrhanical  combination — 
tiie  union  of  atoms  with  tbe  mdon  of  fidlii^  bodies  wiOi 
tbe  earth.  He  calculates  the  Tdocil^  with  which  k 
body  startii^  at  an  infinite  **^«4«»**>  would  stiike  tiie 
eaitik^  sur&ce,  and  finds  that  tibe  heat  genented  by  its 
oollisifln  would  raise  an  equal  weight  of  water  17,356" 
GL  in  tempentme.  He  then  detcnnines  the  thermal 
which  would  be  produced  by  the  earth  itadf  &Ilii^ 

the  son.  So  that  hoe,  in  1845,  we  baTe  the  gena 
of  that  meteoric  tbeoiy  of  the  sm^a  beat  whidi  Mayer 
devd<^ied  with  such  extiaovdinary  alalii^  tibiee 
afterwardsL  He  also  points  to  tbe  almost  exdusive 
caey  of  the  sun^  beat  in  producno^  mechanical 
open  the  earth,  winding  up  with  the  prafiiund 
that  tbe  beat  devel<^»ed  by  finction  in  the  wheds  of  oar 
wind  and  water  mills  comes  from  tibe  sun  In  Idie  fiHon 
of  Tibntoiy  modon ;  wbOe  the  heat  produced  by  miDs 
dii¥€n  by  tidal  action  is  genented  at  the  expense  of  Ae 
earth's  axial  rotation. 

Hkfli^  thuB^  wilh  finn  step^  passed  throi^^  the 
powen  of  inorganic  nature,  bis  next  olgeet  Is  to  bring 
Ida  principleB  to  bear  upon  the  phenomoia  of  Tegetabfe 
md  animal  fife.    Wood  and  coal  can  bum;    aihinin 


434  FRAGMENTS  OF  SCIENCE. 

come  their  heat,  and  the  work  producible  by  that  heat? 
From  the  immeasurable  reservoir  of  the  sun.  Nature 
has  proposed  to  herself  the  task  of  storing  up  the  light 
which  streams  earthward  from  the  sun,  and  of  caating 
into  a  permanent  form  the  most  fugitive  of  all  powers. 
To  this  end  she  has  overspread  the  earth  with  organisms 
which,  while  living,  take  in  the  solar  light,  and  by  its 
consumption  generate  forces  of  another  kind.  These 
organisms  are  plants.  The  vegetable  world,  indeed, 
constitutes  the  instrument  whereby  the  wave-motion  of 
the  sun  is  changed  into  the  rigid  form  of  chemical  ten- 
sion, and  thus  prepared  for  future  use.  With  this  pre- 
vision, as  shall  subsequently  be  shown,  the  existence 
of  the  human  race  itself  is  inseparably  connected.  It 
is  to  be  observed  that  Mayer's  utterances  are  far  from 
being  anticipated  by  vague  statements  regarding  the 
'  stimulus  '  of  light,  or  regarding  coal  as  '  bottled  sun- 
light.' He  first  saw  the  full  meaning  of  De  Saussure's 
observation  as  to  the  reducing  power  of  the  solar  rays,  and 
gave  that  observation  its  proper  place  in  the  doctrine  of 
conservation.  In  the  leaves  of  a  tree,  the  carbon  and 
oxygen  of  carbonic  acid,  and  the  hydrogen  and  oxygen 
of  water,  are  forced  asunder  at  the  expense  of  the  sun, 
and  the  amount  of  power  thus  sacrificed  is  accurately 
restored  by  the  combustion  of  the  tree.  The  heat  and 
work  potential  in  our  coal  strata  are  so  much  strength 
withdrawn  from  the  sun  of  former  ages.  Mayer  lays  the 
axe  to  the  root  of  the  notions  regarding  '  vital  force ' 
which  were  prevalent  when  he  wrote.  With  the  plain 
fact  before  us  that  in  the  absence  of  the  solar  rays 
plants  cannot  perform  the  work  of  reduction,  or  generate 
chemical  tensions,  it  is,  he  contends,  incredible  that  these 
tensions  should  be  caused  by  the  mystic  play  of  the  vital 
force.  Such  an  hypothesis  would  cut  off  all  investiga- 
tion ;  it  would  land  us  in  a  chaos  of  unbridled  phantasy- 


THE  COPLEY  MEDALIST   OF   1871-  435 

'  I  count,'  he  says,  *  therefore,  upori  your  agreement  with 
me  when  I  state,  as  an  axiomatic  truth,  that  during 
vital  processes  the  conversion  only,  and  never  the 
creation  of  matter  or  force  occurs.' 

Having  cleared  his  way  through  the  vegetable  world, 
as  he  had  previously  done  through  inorganic  nature, 
Mayer  passes  on  to  the  other  organic  kingdom.  The 
physical  forces  collected  by  plants  become  the  property 
of  animals.  Animals  consume  vegetables,  and  cause 
them  to  reunite  with  the  atmospheric  oxygen.  Animal 
heat  is  thus  produced  ;  and  not  only  animal  heai,  but 
animal  motion.  There  is  no  indistinctness  about  Mayer 
here;  he  grasps  bis  subject  in  all  its  details,  and  reduces 
to  figures  the  concomitants  of  muscular  action.  A 
bowler  who  imparts  to  an  8-lb.  ball  a  velocity  of  30  feet, 
consumes  in  tlie  act  -^  of  a  grain  of  carbon.  A  man 
weighing  150  lbs.,  who  lifts  his  own  body  to  a  height 
of  8  feet,  consumes  in  the  act  1  grain  of  carbon.  In 
climbing  a  mountain  10,000  feet  high,  the  consumption 
of  the  same  man  would  be  2  oz.  4  drs.  50  grs.  of  carbon. 
Boussingault  had  determined  experimentally  the  ad- 
dition to  be  made  to  the  food  of  horses  when  actively 
working,  and  Liebig  had  determined  the  addition  to  be 
made  to  the  food  of  men.  Employing  the  mechanical 
equivalent  of  heat,  which  he  had  previously  calculated, 
Mayer  proves  the  additional  food  to  be  amply  sufficient 
to  cover  the  increased  oxidation. 

But  he  does  not  content  himself  with  showing,  in  a 
general  way,  that  the  human  body  burns  according  to 
definite  laws,  when  it  performs  mechanical  work.  He 
seeks  to  determine  the  particular  portion  of  the  body 
consumed,  and  in  doing  so  executes  some  noteworthy 
calculations.  The  muscles  of  a  labourer  150  lbs.  in 
weight  weigh  64  lbs. ;  but  when  perfectly  desiccated  they 
fall  to  15  lbs,     Were  the  oxidation  corresponding  to 


436  FKAGMENTS  OF  SCIENCE. 

that  labourer's  work  exerted  on  the  muscles  alone,  they 
would  be  utterly  consumed  in  80  days.  The  heart 
furnishes  a  still  more  striking  example.  Were  the 
oxidation  necessary  to  sustain  the  heart's  action  exerted 
upon  its  own  tissue,  it  would  be  utterly  consumed  in  8 
days.  And  if  we  confine  our  attention  to  the  two 
ventricles,  their  action  would  be  sufficient  to  consume 
the  associated  muscular  tissue  in  3  J  days.  Here,  in  his 
own  words,  emphasised  in  his  own  way,  is  Mayer's 
pregnant  conclusion  from  these  calculations:  'The 
muscle  is  only  the  apparatus  by  means  of  which  the 
conversion  of  the  force  is  effected;  hut  it  is  not  the 
substance  consumed  in  the  production  of  the  mecha- 
nical effect  J  He  calls  the  blood  '  the  oil  of  the  lamp  of 
life;*  it  is  the  slow-burning  fluid  whose  chemical  force, 
in  the  furnace  of  the  capillaries,  is  sacrificed  to  produce 
animal  motion.  This  was  Mayer's  conclusion  twenty-six 
years  ago.  It  was  in  complete  opposition  to  the  scien- 
tific conclusions  of  his  time ;  but  eminent  investigators 
have  since  amply  verified  it. 

Thus,  in  baldest  outline,  I  have  sought  to  give  some 
notion  of  the  first  half  of  this  marvellous  essay.  The 
second  half  is  so  exclusively  physiological  that  I  do  not 
wish  to  meddle  with  it.  I  will  only  add  the  illustration 
employed  by  Mayer  to  explain  the  action  of  the  nerves 
upon  the  muscles.  As  an  engineer,  by  the  motion  of  his 
finger  in  opening  a  valve  or  loosing  a  detent,  can  liberate 
an  amount  of  mechanical  motion  almost  infinite  com- 
pared with  its  exciting  cause,  so  the  nerves,  acting  upon 
the  muscles,  can  unlock  an  amount  of  activity,  wholly 
out  of  proportion  to  the  work  done  by  the  nerves  them- 
selves. 

As  regards  these  questions  of  weightiest  import  to  the 
science  of  physiology.  Dr.  Mayer,  in  1 845,  was  assuredly 
far  in  advance  of  all  living  men. 


THE  COPLEY  MEDALIST  OF   1871.  437 

Mayer  grasped  the  mechanical  theory  of  heat  with 
commanding  power,  illustrating  it  and  applying  it  in 
the  most  diverse  domains.  He  began,  as  we  have  seen, 
with  physical  principles  ;  he  determined  the  numerica.1 
relation  between  heat  and  work ;  he  revealed  the  source 
of  the  energies  of  the  vegetable  world,  and  showed  the 
relationship  of  the  heat  of  our  fires  to  solar  heat.  He 
followed  the  energies  which  were  potential  in  the  vege- 
table, up  to  their  local  exhaustion  in  the  animal.  But 
in  1845  a  new  thought  was  forced  upon  him  by  his 
calculations.  He  then,  for  the  first  time,  drew  attention 
to  the  astounding  amount  of  heat  generated  by  gravity 
where  the  force  has  sufficient  distance  to  act  through. 
He  proved,  as  I  have  before  stated,  the  heat  of  collision 
of  a  body  falling  from  an  infinite  distance  to  the  earth, 
to  be  sufficient  to  raise  the  temperature  of  a  quantity 
of  water,  equal  to  the  falling  body  in  weight,  17,356°  C. 
He  also  found,  in  1845,  that  the  gravitating  force 
between  the  earth  and  sun  was  competent  to  generate 
an  amount  of  heat  equal  to  that  obtainable  from  the 
combustion  of  6,000  times  the  weight  of  the  earth  of 
solid  coal.  With  the  quickness  of  genius  he  saw  that 
we  had  here  a  power  sufficient  to  produce  the  enormous 
temperature  of  the  sun,  and  also  to  account  for  the 
primal  molten  condition  of  our  own  planet.  Mayer 
shows  the  utter  inadequacy  of  chemical  forces,  as  we 
know  them,  to  produce  or  maintain  the  solar  temperature. 
He  shows  that  were  the  sun  a  lump  of  coal  it  would  be 
utterly  consumed  in  5,000  years.  He  shows  the  diffi- 
culties attending  the  assumption  that  the  sun  is  a  cooling 
body;  for,  supposing  it  to  possess  even  the  high  specific 
heat  of  water,  its  temperature  would  fall  15,000°  in 
5,000  years.  He  finally  concludes  that  the  light  and 
beat  of  the  sun  are  maintained  by  the  constant  impact 
of  meteoric  matter.     I  never  ventured  an  opinion  as  to 


438  FRAaMP:NTS  OF  SCIENCE, 

the  truth  of  this  theory ;  that  is  a  question  which 
may  still  have  to  be  fought  out.  But  I  refer  to  it  as 
an  illustration  of  the  force  of  genius  with  which  Mayer 
followed  the  mechanical  theory  of  heat  through  all  its 
applications.  Whether  the  meteoric  theory  be  a  matter 
of  fact  or  not,  with  him  abides  the  honour  of  proving 
to  demonstration  that  the  light  and  heat  of  suns  and 
stars  may  be  originated  and  maintained  by  the  collisions 
of  cold  planetary  matter. 

It  is  the  man  who  with  the  scantiest  data  could 
accomplish  all  this  in  six  short  years,  and  in  the  hours 
snatched  from  the  duties  of  an  arduous  profession,  that 
the  Royal  Society,  in  1871,  crowned  with  its  highest 
honour. 

Comparing  this  brief  history  with  that  of  the  Copley 
Medalist  of  1870,  the  differentiating  influence  of 
*  environment,'  on  two  minds  of  similar  natural  cast 
and  endowment,  comes  out  in  an  instructive  manner. 
Withdrawn  from  mechanical  appliances,  Mayer  fell  back 
upon  reflection,  selecting  with  marvellous  sagacity,  from 
existing  physical  data,  the  single  result  on  which  could 
be  founded  a  calculation  of  the  mechanical  equivalent 
of  heat.  In  the  midst  of  mechanical  appliances.  Joule 
resorted  to  experiment,  and  laid  the  broad  and  firm 
foundation  which  has  secured  for  the  mechanical  theory 
the  acceptance  it  now  enjoys.  A  great  portion  of  Joule's 
time  was  occupied  in  actual  manipulation;  freed  from 
this,  Mayer  had  time  to  follow  the  theory  into  its  most 
abstruse  and  impressive  applications.  With  their  places 
reversed,  however.  Joule  might  have  become  Mayer,  and 
Mayer  might  have  become  Joule. 

It  does  not  lie  within  the  scope  of  these  brief  articles 
to  enter  upon  the  developments  of  the  Dynamical 
Theory  accomplished  since  Joule  and  Mayer  executed 
their  memorable  labours. 


XXI. 
DEATH  BY  LIGUTXINQ, 

PEOPLE  in  general  imagine,  when  they  think  at  all 
about  the  matter,  that  an  impression  upon  the 
nerves— a  blow,  for  example,  or  the  prick  of  a  pin — is 
felt  at  the  moment  it  is  inflicted.  But  this  is  not  the  case. 
The  seat  of  sensation  being  the  brain,  to  it  the  intelli- 
gence of  any  impression  made  upon  the  nerves  has  to  be 
transmitted  before  this  impression  can  become  manifest 
as  consciousness.  The  transmission,  moreover,  requires 
time^  and  the  consequence  is,  that  a  wound  inflicted  on 
a  portion  of  the  body  distant  from  the  brain  is  more 
tardily  appreciated  than  one  inflicted  adjacent  to  the 
brain.  By  an  extremely  ingenious  experimental  arrange- 
ment, Helmholtz  has  determined  the  velocity  of  this 
nervous  transmission,  and  finds  it  to  be  about  eighty 
feet  a  second,  or  less  than  one-thirteenth  of  the  velocity 
of  sound  in  air.  If  therefore,  a  whale  forty  feet  long 
were  wounded  in  the  tail,  it  would  not  be  conscious  of 
the  injury  till  half  a  second  after  the  wound  had  been 
inflicted.^  But  this  is  not  the  only  ingredient  in  the 
delay.  There  can  scarcely  be  a  doubt  that  to  every  act 
of  consciousness  belongs  a  deteiminate  molecular  arrange- 
ment of  the  brain — that  every  thought  or  feeling  has- 

•  A  most  admirable  lecture  on  the  velocity  of  nervous  trans- 
mission has  been  published  by  Dr.  Du  Bois  Raymond  in  the  'Pro- 
ceedings of  the  Royal  Institution '  for  1866,  vol.  iv.  p.  576. 

29 


440  FEAGMENTS  OF  SCIENCE. 

its  physical  correlative  in  that  organ;  and  nothing  can 
be  more  certain  than  that  every  physical  change,  whe- 
ther molecular  or  mechanical,  requires  time  for  its 
accomplishment.  So  that,  besides  the  interval  of  trans- 
mission, a  still  further  time  is  necessary  for  the  brain 
to  put  itself  in  order — for  its  molecules  to  take  up  the 
motions  or  positions  necessary  to  the  completion  of 
consciousness.  Helmholtz  considers  that  one-tenth  of 
a  second  is  demanded  for  this  purpose.  Thus,  in  the 
case  of  the  whale  above  supposed,  we  have  first  half  a 
second  consumed  in  the  transmission  of  the  intelligence 
through  the  sensor  nerves  to  the  head,  one-tenth  of  a 
second  consumed  by  the  brain  in  completing  the  arrange- 
ments necessary  to  consciousness,  and,  if  the  velocity  of 
transmission  through  the  motor  be  the  same  as  that 
through  the  sensor  nerves,  half  a  second  in  sending  a 
command  to  the  tail  to  defend  itself.  Thus  one  second 
and  a  tenth  would  elapse  before  an  impression  made 
upon  its  caudal  nerves  could  be  responded  to  by  a  whale 
forty  feet  long. 

Now,  it  is  quite  conceivable  that  an  injury  might 
be  inflicted  so  rapidly  that  within  the  time  required  by 
the  brain  to  complete  the  arrangements  necessary  to 
consciousness,  its  power  of  arrangement  might  be  des- 
troyed. In  such  a  case,  though  the  injury  might  be  of 
a  nature  to  cause  death,  this  would  occur  without  pain, 
Death  in  this  case  would  be  simply  the  sudden  negation 
of  life,  without  any  intervention  of  consciousness  what- 
ever. 

The  time  required  for  a  rifle-bullet  to  pass  clean 
through  a  man's  head  may  be  roughly  estimated  at  a 
thousandth  of  a  second.  Here,  therefore,  we  should 
have  no  room  for  sensation,  and  death  would  be  pain- 
less. But  there  are  other  actions  which  far  transcend 
in  rapidity  that  of  the  rifle-bullet.     A  flash   of  light- 


DEATH  BY  LIGHTNING.  441 

ning  cleaves  a  cloud,  appearing  and  disappearing  in 
less  than  a  hundred-thousandth  of  a  second,  and  the 
velocity  of  electricity  is  such  as  would  carry  it  in  a 
single  second  over  a  distance  almost  equal  to  that  which 
separates  the  earth  and  moon.  It  is  well  known  that  a 
luminous  impression  once  made  upon  the  retina  endiures 
for  ahout  one- sixth  of  a  second,  and  that  this  is  the 
reason  why  we  see  a  continuous  band  of  light  when  a. 
glowing  coal  is  caused  to  pass  rapidly  through  the  air. 
A  body  illuminated  by  an  instantaneous  flash  continues 
to  be  seen  for  the  sixth  of  a  second  after  the  flash  has 
become  extinct ;  and  if  the  body  thus  illuminated  be  in 
motion,  it  appears  at  rest  at  the  place  where  the  flash 
falls  upon  it.  When  a  colour-top  with  differently-coloured 
sectors  is  caused  to  spin  rapidly  the  colours  blend  together. 
Such  a  top,  rotating  in  a  dark  room  and  illuminated 
by  an  electric  spark,  appears  motionless,  each  distinct 
colour  being  clearly  seen.  Professor  Dove  has  foimd 
that  a  flash  of  lightning  produces  the  same  effect. 
During  a  thunderstorm  he  put  a  colour-top  in  exceed- 
ingly rapid  motion,  and  found  that  every  flash  revealed 
the  top  as  a  motionless  object  with  its  colours  distinct. 
If  illuminated  solely  by  a  flash  of  lightning,  the  naotion 
of  all  bodies  on  the  earth's  surface  would,  as  Dove  has 
remarked,  appear  suspended.  A  cannon-ball,  for  ex- 
ample, would  have  its  flight  apparently  arrested,  and 
would  seem  to  hang  motionless  in  space  as  long  as  the 
luminous  impression  which  revealed  the  ball  remained 
upon  the  eye. 

If,  then,  a  rifle-bullet  move  with  sufficient  rapidity 
to  destroy  life  without  the  interposition  of  sensation, 
much  more  is  a  flash  of  lightning  competent  to  produce 
this  effect.  Accordingly,  we  have  well-authenticated 
cases  of  people  being  struck  senseless  by  lightning  who, 


442  FKAGMENTS  OF  SCIENCE, 

on  recovery,  had  no  memory  of  pain.     The  following 
circumstantial  case  is  described  by  Hemmer : — 

On  June  30,  1788,  a  soldier  in  the  neighbom-hood 
of  Mannheim,  being  overtaken  by  rain,  placed  himself 
under  a  tree,  beneath  which  a  woman  had  previously 
taken  shelter.  He  looked  upwards  to  see  whether  the 
branches  were  thick  enough  to  afford  the  required  pro- 
tection, and,  in  doing  so,  was  struck  by  lightning,  and 
fell  senseless  to  the  earth.  The  woman  at  his  side  ex- 
perienced the  shock  in  her  foot,  but  was  not  struck 
down.  Some  hours  afterwards  the  man  revived,  but 
remembered  nothing  about  what  had  occurred,  save  the 
fact  of  his  looking  up  at  the  branches.  This  was  his 
last  act  of  consciousness,  and  he  passed  from  the  con- 
scious to  the  unconscious  condition  without  pain.  The 
visible  marks  of  a  lightning  stroke  are  usually  insigni- 
ficant :  the  hair  is  sometimes  burnt ;  slight  wounds  are 
observed  ;  while,  in  some  instances,  a  red  streak  marks 
the  track  of  the  discharge  over  the  skin. 

Under  ordinary  circumstances,  the  discharge  from 
a  small  Leyden  jar  is  exceedingly  unpleasant  to  me. 
Some  time  ago  I  happened  to  stand  in  the  presence  of 
a  numerous  audience,  with  a  battery  of  fifteen  large 
Leyden  jars  charged  beside  me.  Through  some  awk- 
wardness on  my  part,  I  touched  a  wire  leading  from  the 
l)attery,  and  the  discharge  went  through  my  body. 
Life  was  absolutely  blotted  out  for  a  very  sensible 
interval,  without  a  trace  of  pain.  In  a  second  or  so  con- 
sciousness returned ;  I  vaguely  discerned  the  audience 
and  apparatus,  and,  by  the  help  of  these  external 
appearances,  immediately  concluded  that  I  had  received 
the  battery  discharge.  The  intellectual  consciousness 
of  my  position  was  restored  with  exceeding  rapidity, 
but  not  so  the  optical  consciousness.  To  prevent  the 
audience  from  being  alarmed,  I  observed  that  it  had 


DEATH   BY  LIGIITNINO.  443 

often  been  my  desire  to  receive  accidentally  such  a 
shock,  and  that  my  wish  had  at  length  been  fulfilled. 
But,  while  making  this  remark,  the  appearance  which 
my  body  presented  to  my  eyes  was  that  of  a  number 
of  separate  pieces.  The  arms,  for  example,  were 
detached  from  the  trunk,  and  seemed  suspended  in  the 
air.  In  fact,  memory  and  the  power  of  reasoning 
appeared  to  be  complete  long  before  the  optic  nerve 
was  restored  to  healthy  action.  But  what  I  wish  chiefly 
to  dwell  upon  here  is,  the  absolute  painlessness  of  the 
shock  ;  and  there  cannot,  I  think,  be  a  doubt  that,  to  a 
person  struck  dead  by  lightning,  the  passage  from  life 
to.  death  occurs  without  consciousness  being  in  the  least 
degree  implicated.  It  is  an  abrupt  stoppage  of  sensa- 
tion, unaccompanied  by  a  pang. 


444  FIIAGMENTS  OF  SCIENCE. 


XXII. 
SCIENCE  AND  THE  'SPIRITS: 

THEIR  refusal  to  investigate  '  spiritual  phenomena  * 
is  often  urged  as  a  reproach  against  scientific  men. 
I  here  propose  to  give  a  sketch  of  an  attempt  to  apply 
to  the  '  phenomena '  those  methods  of  enquiry  which 
are  found  available  in  dealing  with  natural  truth. 

Some  years  ago,  when  the  spirits  were  particularly 
active  in  this  country,  F'araday  was  invited,  or  rather 
entreated,  by  one  of  his  friends  to  meet  and  question 
them.  He  had,  however,  already  made  their  acquaint- 
ance, and  did  not  wish  to  renew  it.  I  had  not  been 
so  privileged,  and  he  therefore  kindly  arranged  a 
transfer  of  the  invitation  to  me.  The  spirits  themselves 
named  the  time  of  meeting,  and  I  was  conducted  to 
the  place  at  the  day  and  hour  appointed. 

Absolute  unbelief  in  the  facts  was  by  no  means  my 
condition  of  mind.  On  the  contrary,  I  thought  it  pro- 
bable that  some  physical  principle,  not  evident  to  the 
spiritualists  themselves,  might  underlie  their  manifes- 
tations. Extraordinary  effects  are  produced  by  the 
accumulation  of  small  impulses.  Galileo  set  a  heavy 
pendulum  in  motion  by  the  well-timed  puffs  of  his 
breath.  Ellicot  set  one  clock  going  by  the  ticks  of 
another,  even  when  the  two  clocks  were  separated  by  a 
wall.  Preconceived  notions  can,  moreover,  vitiate,  to 
an  extraordinary  degree,  the  testimony  of  even  veracious 


SCIENCE  AND  THE   'SPIRITS.'  445 

persons.  Hence  my  desire  to  witness  those  extra- 
ordinary phenomena,  the  existence  of  which  seemed 
placed  beyond  a  doubt  by  the  known  veracity  of  those 
who  had  witnessed  and  described  them.  The  meeting 
took  place  at  a  private  residence  in  the  neighbourhood 
of  London.  My  host,  his  intelligent  wife,  and  a  gentle- 
man who  may  be  called  X.,  were  in  the  house  when  I 
arrived.  I  was  informed  that  the  '  medium '  had  not 
yet  made  her  appearance ;  that  she  was  sensitive,  and 
might  resent  suspicion.  It  was  therefore  requested 
that  the  tables  and  chairs  should  be  examined  before 
her  arrival,  in  order  to  be  assured  that  there  was  no 
trickery  in  the  furniture.  This  was  done  ;  and  I  then 
first  learned  that  my  hospitable  host  had  arranged  that 
the  seance  should  be  a  dinner-party.  This  was  to  me 
an  unusual  form  of  investigation ;  but  I  accepted  it, 
as  one  of  the  accidents  of  the  occasion. 

The  'medium'  arrived — a  delicate-looking  young 
lady,  who  appeared  to  have  suffered  much  from  ill- 
health.  I  took  her  to  dinner  and  sat  close  beside  her. 
Facts  were  absent  for  a  considerable  time,  a  series  of 
very  wonderful  narratives  supplying  their  place.  The 
duty  of  belief  on  the  testimony  of  witnesses  was  frequently 
insisted  on.  X.  appeared  to  be  a  chosen  spiritual  agent, 
and  told  us  many  surprising  things.  He  affirmed  that, 
when  he  took  a  pen  in  his  hand,  an  influence  ran  from 
his  shoulder  downwards,  and  impelled  him  to  write  oracu- 
lar sentences.  I  listened  for  a  time,  offering  no  observa- 
tion. 'And  now,'  continued  X.,  'this  power  has  so 
risen  as  to  reveal  to  me  the  thoughts  of  others.  Only 
this  morning  I  told  a  friend  what  he  was  thinking  of, 
and  what  he  intended  to  do  during  the  day,'  Here,  I 
thought,  is  something  that  can  be  at  once  tested.  I 
said  immediately  to  X. :  '  If  you  wish  to  win  to  your 
cause  an  apostle,  who  will  proclaim  your  principles  to 


446  FKAGMENTS  OF  SCIENCE. 

the  world  from  the  housetop,  tell  nie  what  I  am  no^v 
thinking  of.*  X.  reddened,  and  did  not  tell  me  my 
thought. 

Some  time  previously  I  had  visited  Baron  Eeichen- 
bach,  in  Vienna,  and  I  now  asked  the  young  lady  who 
sat  beside  me,  whether  she  could  see  any  of  the  curious 
things  which  he  describes — the  light  emitted  by  crystals, 
for  example  ?  Here  is  the  conversation  which  followed, 
as  extracted  from  my  notes,  written  on  the  day  fol- 
lowing the  seance. 

Medium, — 'Oh,  yes;  but  I  see  light  around  all 
bodies,' 

/. — ^Even  in  perfect  darkness  ?' 

Medium. — '  Yes ;  I  see  luminous  atmospheres 
round  all  people.  The  atmosphere  which  surrounds 
Mr.  E.  C.  would  fill  this  room  with  light.' 

/. — '  You  are  awn  re  of  the  effects  ascribed  by  Baron 
Reichenbach  to  magnets  ? ' 

Medium, — '  Yes ;  but  a  magnet  makes  me  terribly 
ill.* 

/. — *Am  I  to  understand  that,  if  this  room  were 
perfectly  dark,  you  could  tell  whether  it  contained  a 
magnet,  without  being  informed  of  the  fact  ? ' 

Medium. — 'I  should  know  of  its  presence  on 
entering  the  room.' 

/.—'How?' 

Medium. — '  I  should  be  rendered  instantly  ill. 

7. — '  How  do  you  feel  to-d;iy  ?  ' 

Medium.. — '  Particularly  well ;  I  have  not  been  so 
well  for  months.' 

/. — '  Then,  may  I  ask  you  whether  there  is,  at  the 
present  moment,  a  magnet  in  my  possession  ? ' 

The  young  lady  looked  at  me,  blushed,  and  stam- 
mered, 

'  No  ;  I  am  not  en  rapport  with  you.' 


SCIENCE  AND  THE     SPIRITS/  447 

I  sat  at  her  right  hand,  and  a  left-hand  pocket, 
within  six  inches  of  her  person,  contained  a  magnet. 

Our  host  here  deprecated  discussion,  as  it '  exhausted 
the  medium.'  The  wonderful  narratives  were  resumed ; 
but  I  had  narratives  of  my  own  quite  as  wonderful. 
These  spirits,  indeed,  seemed  clumsy  creations,  com- 
pared with  those  with  which  my  own  work  had  made 
me  familiar.  I  therefore  began  to  match  the  wonders 
related  to  me  by  other  wonders.  A  lady  present  dis- 
coursed on  spiritual  atmospheres,  which  she  could  see 
as  beautiful  colours  when  she  closed  her  eyes.  I  pro- 
fessed myself  able  to  see  similar  colours,  and,  more 
than  that,  to  be  able  to  see  the  interior  of  my  own 
eyes.  The  medium  affirmed  that  she  could  see  actual 
waves  of  light  coming  from  the  sun.  I  retorted  that 
men  of  science  could  tell  the  exact  number  of  waves 
emitted  in  a  second,  and  also  their  exact  length.  The 
medium  spoke  of  the  performances  of  the  spirits  on 
musical  instruments.  I  said  that  such  performance 
was  gross,  in  comparison  with  a  kind  of  music  which 
had  been  discovered  some  time  previously  by  a  scientific 
man.  Standing  at  a  distance  of  twenty  feet  from  a 
jet  of  gas,  he  could  command  the  flame  to  emit  a 
melodious  note  ;  it  would  obey,  and  continue  its  song 
for  hours.  So  loud  was  the  music  emitted  by  the  gas- 
flame,  that  it  might  be  heard  by  an  assembly  of  a 
thousand  people.  These  were  acknowledged  to  be  as 
great  marvels  as  any  of  those  of  spiritdom.  The  spirits 
were  then  consulted,  and  I  was  pronounced  to  be  a 
first-class  medium. 

During  this  conversation  a  low  knocking  was  heard 
from  time  to  time  under  the  table.  These,  I  was  told, 
were  the  spirits'  knocks.  I  was  informed  that  one  knock, 
in  answer  to  a  question,  meant  '  No  ; '  that  two  knocks 
meant '  Not  yet , '  and  that  three  knocks  meant  '  Yes.' 


448  FRAGMENTS  OF  SCIENCE. 

In  answer  to  a  question  whether  I  was  a  medium,  the 
response  was  three  brisk  and  vigorous  knocks.  I  noticed 
that  the  knocks  issued  from  a  particular  locality,  and 
therefore  requested  the  spirits  to  be  good  enough  to 
answer  from  another  corner  of  the  table.  They  did 
not  comply ;  but  I  was  assured  that  they  would  do  it, 
and  much  more,  by-and-by.  The  knocks  contini)ing, 
I  turned  a  wine-glass  upside  down,  and  placed  my  ear 
upon  it,  as  upon  a  stethoscope.  The  spirits  seemed  dis- 
concerted by  the  act ;  they  lost  their  playfulness,  and 
did  not  recover  it  for  a  considerable  time. 

Somewhat  weary  of  the  proceedings,  I  once  threw 
myself  back  against  my  chair  and  gazed  listlessly  out 
of  the  window.  While  thus  engaged,  the  table  was 
rudely  pushed.  Attention  was  drawn  to  the  wine,  still 
oscillating  in  the  glasses,  and  I  was  asked  whether  that 
was  not  convincing.  I  readily  granted  the  fact  of 
motion,  and  began  to  feel  the  delicacy  of  my  position. 
There  were  several  pairs  of  arms  upon  the  table,  and 
several  pairs  of  legs  under  it ;  but  how  was  I,  without 
offence,  to  express  the  conviction  which  I  f^Uy  enter- 
tained ?  To  ward  off  the  difficulty,  I  again  turned  a 
wine-glass  upside  down  and  rested  my  ear  upon  it. 
The  rim  of  the  glass  was  not  level,  and  my  hair,  on 
touching  it,  caused  it  to  vibrate,  and  produce  a  peculiar 
buzzing  sound.  A  perfectly  candid  and  warm-hearted 
old  gentleman  at  the  opposite  side  of  the  table,  whom 
I  may  call  A.,  drew  attention  to  the  sound,  and 
expressed  his  entire  belief  that  it  was  spiritual.  I, 
however,  informed  him  that  it  was  the  moving  hair 
acting  on  the  glass.  The  explanation  was  not  well 
received;  and  X.,  in  a  tone  of  severe  pleasantry, 
demanded  whether  it  was  the  hair  that  had  moved  the 
table.  The  promptness  of  my  negative  probably 
satisfied  him  that  my  notion  was  a  very  different  one. 


SCIENCE  AND   THE  'SPIRITS.'  449 

Tlie  superhuman  power  of  the  spirits  was  next 
dwelt  upon.  The  strength  of  man,  it  was  stated,  was 
unavailing  in  opposition  to  theirs.  No  human  power 
could  prevent  the  table  from  moving  when  they  pulled 
it.  During  the  evening  this  pulling  of  the  table 
occurred,  or  rather  was  attempted,  three  times.  Twice 
the  table  moved  when  my  attention  was  withdrawn 
from  it ;  on  a  third  occasion,  I  tried  whether  the  act 
could  be  provoked  by  an  assumed  air  of  inattention. 
Grrasping  the  table  firmly  between  my  knees,  I  threw 
myself  back  in  the  chair,  and  waited,  with  eyes  fixed 
on  vacancy,  for  the  pull.  It  came.  For  some  seconds 
it  was  pull  spirit,  hold  muscle ;  the  muscle,  however, 
prevailed,  and  the  table  remained  at  rest.  Up  to  the 
present  moment,  this  interesting  fact  is  known  only  to 
the  particular  spirit  in  question  and  myself. 

A  species  of  mental  scene-painting,  with  which  my 
own  pursuits  had  long  rendered  me  familiar,  was 
employed  to  figure  the  changes  and  distribution  of 
spiritual  power.  The  spirits,  it  was  alleged,  were  pro- 
vided with  atmospheres,  which  combined  with  and 
interpenetrated  each  other,  and  considerable  ingenuity 
was  shown  in  demonstrating  the  necessity  of  time  in 
effecting  the  adjustment  of  the  atmospheres.  A  re- 
arrangement of  our  positions  was  proposed  and  carried 
out ;  and  soon  afterwards  my  attention  was  drawn  to  a 
scarcely  sensible  vibration  on  the  part  of  the  table. 
Several  persons  were  leaning  on  the  table  at  the  time, 
and  I  asked  permission  to  touch  the  medium's  hand. 
'  Oh  I  I  know  I  tremble,'  was  her  reply.  Throwing  one 
leg  across  the  other,  I  accidentally  nipped  a  muscle,  and 
produced  thereby  an  involuntary  vibration  of  the  free 
leg.  This  vibration,  I  knew,  must  be  communicated 
to  the  floor,  and  thence  to  the  chairs  of  all  present. 
r  therefore  intentionally  promoted  it.     JNIy  attention 


450  FRAGMENTS  OF  SCIENCE. 

was  promptly  drawn  to  the  motion  ;  and  a  gentleman 
beside  me,  whose  value  as  a  witness  I  was  particularly 
desirous  to  test,  expressed  his  belief  that  it  was  out  of 
the  compass  of  human  power  to  produce  so  strange  a 
tremor.  '  I  believe,'  he  added,  earnestly,  '  that  it  is 
entirely  the  spirits'  work.'  '  So  do  I,'  added,  with  heat, 
the  candid  and  warmhearted  old  gentleman  A.  'Why, 
sir,'  he  continued,  '  I  feel  them  at  this  moment  shaking 
my  chair.*  I  stopped  the  motion  of  the  leg.  '  Now, 
sir,'  A.  exclaimed,  '  they  are  gone.'  I  began  again,  and 
A.  once  more  affirmed  their  presence.  I  could,  how- 
ever, notice  that  there  were  doubters  present,  who  did 
not  quite  know  what  to  think  of  the  manifestations. 
I  saw  their  perplexity ;  and,  as^  there  was  sufficient 
reason  to  believe  that  the  disclosure  of  the  secret 
would  simply  provoke  anger,  I  kept  it  to  myself. 

Again  a  period  of  conversation  ii^^i'vened,  during 
which  the  spirits  became  animated.  The  evening  was 
confessedly  a  dull  one,  but  matters  appeared  to  brighten 
towards  its  close.  The  spirits  were  requested  to  spell 
the  name  by  which  I  was  known  in  the  heavenly  world. 
Our  host  commenced  repeating  the  alphabet,  and  when 
he  reached  the  letter  'P'  a  knock  was  heard.  He 
began  again,  and  the  spirits  knocked  at  the  letter  '  0.' 
I  was  puzzled,  but  waited  for  the  end.  The  next 
letter  knocked  down  was  '  E.'  I  laughed,  and  remarked 
that  the  spirits  were  going  to  make  a  poet  of  me. 
Admonished  for  my  levity,  I  was  informed  that  the 
frame  of  mind  proper  for  the  occasion  ought  to  have 
been  superinduced  by  a  perusal  of  the  Bible  imme- 
diately before  the  seance.  The  spelling,  however,  went 
on,  and  sure  enough  I  came  out  a  poet.  But  matters 
did  not  end  here.  Our  host  continued  his  repetition 
of  the  alphabet,  and  the  next  letter  of  the  name  proved 
to  be  *  0.'     Here  was  manifestly  an  unfinished  word , 


SCIENCE  AND  THE  'SPIEIia'  ,       451 

and  the  spirits  were  apparently  in  their  most  communi- 
cative mood.  The  knocks  came  from  under  the  table, 
but  no  person  present  evinced  the  slightest  desire  to 
look  under  it.  I  asked  whether  I  might  go  under- 
neath ;  the  permission  was  granted ;  so  I  crept  under 
the  table.  Some  tittered ;  but  the  candid  old  A.  ex- 
claimed, '  He  has  a  right  to  look  into  the  very  dregs  of 
it,  to  convince  himself.'  Having  pretty  well  assured 
myself  that  no  sound  could  be  produced  under  the  table 
without  its  origin  being  revealed,  I  requested  our  host 
to  continue  his  questions.  He  did  so,  but  in  vain. 
He  adopted  a  tone  of  tender  entreaty ;  but  the  '  dear 
spirits  '  had  become  dumb  dogs,  and  refused  to  be 
entreated.  I  continued  under  that  table  for  at  least  a 
quarter  of  an  hour,  after  which,  with  a  feeling  of 
despair  as  regards  the  prospects  of  humanity  never  be- 
fore experienced,  I  regained  my  chair.  Once  there,  the 
spirits  resumed  their  loquacity,  and  dubbed  me  '  Poet 
of  Science.' 

This,  then,  is  the  result  of  an  attempt  made  by  a 
scientific  man  to  look  into  these  spiritual  phenomena. 
It  is  not  encouraging ;  and  for  this  reason.  The  present 
promoters  of  spiritual  plienomena  divide  themselves 
into  two  classes,  one  of  which  needs  no  demonstration, 
while  the  other  is  beyond  the  reach  of  proof.  The 
victims  like  to  believe,  and  they  do  not  like  to  be  un- 
deceived. Science  is  perfectly  powerless  in  the  presence 
of  this  frame  of  mind.  It  is,  moreover,  a  state  per- 
fectly compatible  with  extreme  intellectual  subtlety 
and  a  capacity  for  devising  hypotheses  which  only 
require  the  hardihood  engendered  by  strong  conviction, 
or  by  callous  mendacity,  to  render  them  impregnable. 
The  logical  feebleness  of  science  is  not  sufficiently 
borne  in  mind.  It  keeps  down  the  weed  of  superstition, 
not  by  logic  but  by  slowly  rendering  the  mental  soil 


452  FRAGMENTS  OF  SCIENCE. 

unfit  for  its  cultivation.  When  science  appeals  to 
uniform  experience,  the  spiritualist  will  retort,  '  How 
do  you  know  that  a  uniform  experience  will  continue 
uniform  ?  You  tell  me  that  the  sun  has  risen  for  six 
thousand  years :  that  is  no  proof  that  it  will  rise  to- 
morrow ;  within  the  next  twelve  hours  it  may  he  puffed 
out  by  the  Almighty.'  Taking  this  ground,  a  man 
may  maintain  the  story  of  '  Jack  and  the  Beanstalk '  in 
the  face  of  all  the  science  in  the  world.  You  urge,  in 
vain,  that  science  has  given  us  all  the  knowledge  of  the 
universe  which  we  now  possess,  while  spiritualism  has 
added  nothing  to  that  knowledge.  The  drugged  soul 
is  beyond  the  reach  of  reason.  It  is  in  vain  that  im- 
postors are  exposed,  and  the  special  demon  cast  out. 
He  has  but  slightly  to  cha^e  his  shape,  return  to  hia 
house,  and  find  it  *  empty,  swept,  and  garnished.' 


Since  the  time  when  the  foregoing  remarks  were 
written  I  have  been  more  than  once  among  the  spirits, 
at  their  own  invitation.  They  do  not  improve  on  ac- 
quaintance. Surely  no  baser  delusion  ever  obtained 
dominance  over  the  weak  mind  of  man. 


END   OF  THE   FIRST  VOLUME. 


THE  UNIVERSITY  LIBRARY 

UNIVERSITY  OF  CALIFORNIA,  SANTA  CRUZ 

SCIENCE  LIBRARY 


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